This research investigates formation process of organizational culture to be possible ambidextrous organization. Organizational culture is important as comprehensive context that makes possible organizational ambidexterity. Therefore, this paper includes theoretical backgrounds about ambidextrous organization, organizational culture, and organizational identity as the factor reciprocally influencing on organizational culture. The desirability of the ambidextrous organization was already known. However, the research about the specific process for constructing the ambidextrous organization has been insufficient. This study was conducted using qualitative methods that used interview and archival data collected in 2 periods. This research investigates the organizational culture change starting with new business diversification at ElectroFi (a pseudonym), an interbank financial institution in Korea. Interviews are divided into two times. As to the first interview times, 29 interviews were performed till March in 2002 from December in 2000. As to the second interview times, 13 interviews were performed from 2011 to 2012. This research interviewed informant who could provide rich and insightful information on research phenomenon and chose informants who would be most able to inform us on main research question concerning how ambidextrous organizational culture integrates. At the time of interviewing, most informants have been worked over 10 years at ElectroFi. All informants were full-time employees. The average interview hours lasted an hour and a half. More than 80% of interviews were tape recorded. Data collection for this study ended in March 2012.Information from documents and records including organization historiography, formal and informal reports, and news letter served as important supplements to interview data. Analysis proceeds along the lines of the constant comparative method in which new data are constantly compared to prior to interpretations. Data collection and analysis are guided by emergent design. This study penetrates into a single organizational example throughout the paper. Researcher has spent a substantial amount of time for the purpose of gathering data and developing "thick description". ElectroFi's progressive process could be classified into the stage of traditional non-profit organizational culture from 1986 to 1994, the stage of new profit subculture formation arising from start and stabilization of the business diversification from 1995 to the first half of 2001, and the stage of ambidextrous organizational culture which has originated with the integrating dominant culture with new subculture from the second half of 2001 to 2011. The first stage significantly showed traditional organizational culture. The second stage had formed the new subculture through the business diversification and heterogeneous subculture showed outstandingly. In the third stage, the cultural integration between dominant culture and subculture occurred. And, balancing identity in this stage is important to maintain and build ambidextrous organizational culture. Factors affecting integration between two cultural units are subculture management policy, business relatedness, inter-group contacts. Firstly, management policy to achieve integration needs to maintain autonomous and independent subculture that organizational members would simultaneously accept the core values of the dominant culture and separate subculture. Secondly, business relatedness is necessary condition for the integration between cultural units, because it induces two dimensions to utilize common resources. Thirdly, intergroup contacts are very important to cultural integration between business units. Organizational culture functions as a context for sense-making. Theoretical works in a social constructionist tradition, conversely, have emphasized reciprocal influence of culture and identity in affecting sense making in organizations. Organizational members coexisting in nonprofit and profit identities experienced identity conflict and identity tensions in this period. At ElectroFi, managers try to embed new identity claims in organizational culture for the purpose of decreasing identity conflict and confusion. Ambidextrous organizational culture's characteristics were as follows; Non-profit culture (dominant culture) was traditionally careful and took the serious procedures in comparison with the profit division subculture. The profit subculture pursued continuously the new attempt. Therefore, the non-profit organization's prudent and systematic business procedure united with profit division highly regarding new trial. This combination resulted in the synergy effects in management. Organization progressed with ambidextrous organizational culture at the same time pursuit of exploiting existing knowledge and exploring new idea.