Abstract: Abstract：　Objective To evaluate the microleakage of class I composite resin restorations with and without flowable composite liner. Methods　Cylindrical, Class I, 4-mm× 4-mm ×4-mm preparations were made on 48 recently extracted human molars .The samples were divided into 6 groups randomly.Three groups were applied flowable composite liner,and were filled with Tetric N-Ceram Bulk Fill and Filtek P60 composite resin by two filling techniques respectively.The rest of the three groups were applied no flowable composite liner,and filled with restorations metioned above respectively. All the teeth were kept in methylene blue solution for 24 hours,and were sectioned buccolingual though the midline of the materials and examined with the stereoscopic microscope.Results　The microleakage of group A,B and C was lower than group D,E and F ( P ＜ 0.05) .The microleakage of group A and C was lower than group B ( P ＜ 0.05) ，while there was no significant difference between group A and group C( P >0.05). Conclusions　 Using flowable composite as a liner will reduce microleakage．The bulk-fill technology was the same as the traditional layered filling technology in the microleakage of composite resin restoration.

Key words: Key words：　microleakage , flowable composite resin , liner, bulk fill composite resin