The objective of this study was to evaluate the effects of triethylene glycol (TEG) and triethylene glycol monomethacrylate (TEGMA) solutions as dentin primers on dentin bonding. To this end, wall-to-wall polymerization contraction gap width of a resin composite in a cylindrical dentin cavity and shear bond strength to a flat dentin surface were measured. Dentin was pretreated with an experimental dentin bonding system -consisting of 0.5 M ethylenediaminetetraacetic acid conditioner, TEG or TEGMA primer, and Clearfil Photo Bond bonding agent -prior to resin composite filling. When the cavity was primed with an aqueous solution of 35 vol％ TEG, 35 or 45 vol％ TEGMA for a few seconds, contraction gap formation was prevented completely. Then, among these three gap-free groups, there were no significant differences in shear bond strength. It was thus shown that both TEG and TEGMA were highly effective dentin primers, completely preventing contraction gap formation even when they were applied for only a few seconds.