Today, companies are faced with marketing difficulties both new customer acquisition and existing customer retention due to the market saturation in most of industries. In this case,the author introduces T-membership Cashbag, the co-marketing program between SK Telecom and SK Marketing & Company, and describes background, service offerings, and event promotion activities of the co-marketing program from year of 2009 to 2011. T-membership Cashbag program, which provided additional benefits of OK Cashbag to the SK Telecom membership customers, launched for the purpose of shared customer’s satisfaction enhancement between SK Telecom and SK Marketing & Company. T-membership Cashbag achieved 8million transactions in a year after service launching, and had great success to break through 15 million transactions the year after. This co-marketing case can provide some companies which have diversified business portfolio with co-marketing solutions which can be differentiated from competitors while minimizing additional marketing expenses.