Microleakage usually occurs in deep cavity restoration. Therefore, sandwich restoration technique was proposed to decrease the microleakage. Sandwich restoration is a restoration technique that combines two restoration materials to form a monolithic restoration between the materials and the enamel. The base materials used are flowable resin composite and Smart Dentin Replacement (SDR). This study aimed to know whether there is a difference of microleakage on sandwich restoration using flowable resin composite and Smart Dentin Replacement (SDR) as the base material and using selective-etching adhesive system. This study was using 34 samples, devide into 2 groups prepared in cavity class II. First group was using flowable resin composite as the base material. Second group was using Smart Dentin Replacement (SDR) as the base material and was taking composite resin as the occlusal cuspal material. The teeth placed in pH 6,8 artificial saliva in 370C incubator for 24 hours. Then, the teeth are dried and coated with 1 mm nail polish and the apical are coated with red wax. The teeth placed in 2% of blue methylen for 24 hours. Next, clean the teeth and separate it mesio-distal. Then, observe the teeth using stereomicroscope with 10x magnification. Based on the result of microleakage test, the average result in group 1 is 22.82 and in group 2 is 12.18. The average of test result conclude that there is significant difference and the use of flowable resin composite as the base material in sandwich restoration shows the higher microleakage than the use of Smart Dentin Replacement (SDR) as the base material.