The main objectives of this study are to analyze the market structure and the commercial banks's reaction in housing loan by measuring concentration among Thai commercial banks from 1997 to 2002. In addition, the conjectural variation model is formulated to examine the commercial banks's reaction in housing loan. The result of this study states that market structure of housing loan is Oligopoly. Housing loan market's concentration, however, substantially decrease from 1997 to 2002, showing more competition among banks. Conjectural variation analysis illustrates that there are the reactions among banks to a change in housing loan volume. We find that when a small bank increases its housing loan volume, the other banks react by reducing their housing loan volume. On the other hand, when a large bank increases its housing loan volume, the rest of the competitors increase their housing loan volume to protect their shares. We conclude even though competition has recently increased, the market power of the large commercial banks still remain in housing loan market.