Few large banks dominate the Indonesia banking industri. Furthermore, in the past ten years, there were a series of mergers and acquisitions in the banking market. The facts cause implications on competition. In this paper, we examine these issues exploiting an unconsolidated annual financial report of all commercial banks between 2001 and 2009. The Panzar-Rose method is employed to examine the banks behavior in competition. Estimates indicate that banks in all three subsamples, large; medium-sized and small are working in a monopolistically competitive market. The analysis of market concentration supports the conventional view that concentration impairs competition. The study shows that the most competitive market was the medium-sized bank...