The relation between the bonding efficacy of the dentin adhesive and the physical change of the dentin after treatment with various dentin cleansers was investigated. The dentin surface was cleaned with one of the seven experimental dentin cleansers, 5, 10, 20, 30 and 40% of the phosphoric acid, neutralized 0.5 M EDTA solution (pH 7.4) and 9.1% pyruvic acid containing 9.1% glycine, prior to the combining application of 35% hydroxyethylmethacrylate solution (HEMA) and a marketed dentin bonding agent. It was found that the bonding efficacy of the dentin bonding system employed in this study deteriorated with the degree of reduction of Vickers hardness on the dentin surface after the dentin cleaning.