Marginal adaptation of four flowable composites was evaluated by measuring wall-to-wall contraction gap in a cylindrical dentin cavity. Four combinations of conventional and flowable resin composites were employed. The cylindrical dentin cavities prepared in extracted human molars were restored with one of these composites after pretreatment according to the manufacturers' instructions. For the positive control, the cavity was conditioned with EDTA, primed with 35% glyceryl mono-methacrylate (GM), and Clearfil Photo Bond was applied prior to the composite filling. Ten specimens for each material, 160 in total, were examined. In the positive control groups, marginal adaptation was completed with the exception of the Unifil-S specimen. With the commercial bonding systems, gaps were observed in two some specimens out of 10. Four flowable resin composites exhibited comparable marginal adaptation compared to the conventional high-viscous composite though complete marginal integrity could not be obtained when the cavity was pretreated with the commercial dentin bonding systems. It was concluded that the flowable resin composite would be clinically useful in combination with a highly effective experimental dentin bonding system composed of EDTA, GM and Clearfil Photo Bond.