Physical changes in the surface characteristics of dentin during bonding procedures were evaluated by measuring the contact angles of three standard liquids, to determine the surface free energy, and that of a commercially available dual cured dentin bonding agent dropped on the primed dentin surface. The commercial dentin bonding agent did not form a hemispherical shape on the ground dentin surface, probably because it penetrated into the smear layer due to the microcapillary effect. Not could the contact angle be measured on the dentin surfaces treated with any of four experimental primers, because the bonding agent rapidly spread over the primed dentin surface. It was concluded that the priming of the dentin surface after removing the smear layer served to increase the surface free energy and to improve the wettability of the bonding agent on the dentin.