The purpose of present study was to evaluate the influence of different base materials on the marginal integrity of posterior direct composite resin restorations. Conventional mesio-occluso-distal(MOD) cavity preparations, with margins in enamel and dentin, were prepared in 24 extracted human mandibular molars and randomly divided into four equal groups. One of three base materials (RMGI, compomer, flowable resin) was placed on the pulpal floors of the teeth of the groups. One group of teeth, which served as the control group, was not given any base material. Then, all teeth were restored with composite resin. The micromorphology of the tooth/restoration interfaces along the entire surfaces of the restorations was quantitatively analyzed using microscope at 150 magnifications immediately after finishing and after completion of thermo-mechanical stress. Marginal adaptation was assessed along the entire margin and % perfect margin (%PM) was calculated. The %PMs before and after thermo-mechanical stress in the groups were compared with Two- way ANOVA with Tukey test at the 95% confidence level. Before thermo-mechanical stress, there was no difference in marginal adaptation between groups. After the thermo-mechanical stress, the RMGI group showed the highest marginal adaptation rate among the tested groups. In compomer and flowable resin groups, the marginal adaptation was lower after thermomechanical stresses than before the stress (p<0.05).