To examine the microleakage of posterior esthetic restorative materials, using 2% methylene blue dye solution, dye penetration was determined with 80 permanent posterior teeth after thermocycling. The following results were obtained. 1. Amalgam group showed the most severe dye penetration of all test groups through the enamel and dentin / cementum margins. 2. Clearfil group showed much more dye penetration than P-50 at the enamel margin and than resin inlay system at the dentin / cementum margin. 3. Through the etched enamel margin, P-50 and resin inlay system groups showed no dye penetration, if any, but through the dentin / cementum margin, all test groups showed dye penetration. 4. Resin inlay system showed the least dye penetration of all test groups through the dentin / cementum margin.