Objective:To investigate the marginal microleakage after cavity preparation by Er:YAG la-ser and conventional bur applying different adhesive systems , conditioning surface with acid etching or not.Methods:In the study, 50 primary teeth were divided into 5 groups.(1) Bur+etch-and-rinse sys-tem, (2) Bur+self-etching system, (3) Er:YAG laser+etch-and-rinse system, (4) Er:YAG laser+self-etching system, (5) Er:YAG laser+none-etched.The class V cavities were all filled with composite resin.The microleakage tests were performed , observed and scored under stereomicroscope .Results:Bur+etch-and-rinse group had higher microleakage than Er:YAG laser +etch-and-rinse group , and Bur+self-etching group had higher microleakage than Er:YAG laser +self-etching group ( P<0 .05 ) . There was no difference between Er:YAG laser+etch-and-rinse group and Er:YAG laser+self-etching group, while they had significantly lower microleakage than that of Er:YAG+none-etched group.Con-clusion:Composite resin restoration showed less marginal microleakage when all-in-one self-etch system was used after irradiation by Er:YAG laser on primary teeth .