The CoMo/Al2O3 catalysts with different metal loading were studied by low temperature in situ FT-IR using CO as probe molecule which appears to be a powerful method by giving rise to signals specific for unpromoted and promoted Mo sites. The result revealed that the increase of C0M0S phase on the catalyst surface improves the HDS activity and selectivity. The ratio of active site number of CoMoS and MoS2 correlates linearly with HDS selectivity, which provides an effective tool for developing industrial selective HDS catalysts.