The objective of the present study was to understand the role of the functional monomers in dentin bonding agents of an experimental dentin bonding system by measuring the wall-to-wall contraction gap and tensile bond strength measurement. The efficacy of three commercial dentin bonding agents after using EDTA for conditioning and GM for priming was evaluated by measuring the contraction gap of the resin composite in a cylindrical dentin cavity, and by measuring the tensile bond strength of the composite to a flat dentin surface. The effect of the functional monomers was demonstrated by the contraction gap measurement alone. The value of the contraction gap was significantly different between the commercial dentin bonding agents and these agents without functional monomers (p<0.05). It was concluded that the functional monomers were essential to obtaining the marginal integrity of the resin composite in the dentin cavities.