Background and Objective: Microleakage is the most important reason for secondary caries and treatment failure. The aim of this study was to evaluate the effect of liner (flowable composite) and bonding agents on microleakage of resin composite (z₂₅₀₎ in class II cavities.

Materials and Methods: Standardized Class II cavities were prepared on mesial and distal of 80 human premolars. The teeth were divided into eight groups based on the position of gingival marginal, bonding agent and liner. Thus, the classification of groups were as follows: group I: margin (1mm sub CEJ), bonding (single bond,3M), and resin composite(z 250,3M), group II: flowable composite (filtek flow, 3M) as a liner, the rest similar to group I. group III: margin (1mm supra CEJ), the rest similar to group I, group IV: margin (1mm supra CEJ), the rest similar to group II, group V: bonding (Adper easy bond,self-etch,3M), the rest similar to group I, group VI: bonding (self-etch), the rest similar to group II, group VII: self-etching bonding, the rest similar to group III, and group VIII: self-etching bonding, the rest similar to group IV. All the samples were thermocycled (3000 times) and  placed in 10% metylen blue dye, then  sectioned with diamond disk. They were evaluated using a stereomicroscope (40 x). The data were analyzed by one way variance and tuky test (p<0.05)

Results: Concerning sub-CEJ margin, the greatest microleakage was in group I and the least was in group VI (P>0.05). Concerning supra-CEJ margin, group III had the greatest microleakage while group VIII had the least.(p>0.05). The comparison between groups showed significant differences between groups I and VIII (p<0.05)

Conclusion: Combination of self-etch adhesive with flowable resin composite (liner) as a packable resin composite z₂₅₀ decreases microleakage.