Self-serviced online banking system (OBS) has transformed the banking services to 24hrs/7days available online. The importance of online banking service and its impacts on users and the banking industry have been widely discussed. However, the retention of loyal users in the sense about how to motivate light users to become heavy users and its link with the confirmation of OBS quality are overlooked. This paper introduces information system (IS) success factors and the concept of commitment assurance to empirically investigate OBS users' continued to use intentions. Results reveal that OBS system and service qualities are significantly important as they are antecedences of OBS user satisfaction. Heavy users show their self-interests in maintaining good relationship with the bank and the differences between heavy and light users are confirmed. Discussions and suggestions for promoting online banking service are given.