Background and aim: Given the prevalence of microleakage in composites and their known complications and lack of reporting of the influence of cavity depth and diameter on the same C-factor on the microleakage of composite cavities, this study investigated the effect of microleakage on composite cavities in identical C Factors. Materials and Methods: In this experimental study on 40 specimen (4 group of 10 each ) Cylindrical cavities with different volumes were prepared using diamond bur no. 006 in buccal surface of teeth (cfactor = 5). Height and width of cavities, respectively: Group I: Height and diameter 2 mm. groupII: height 2 and diameter 4 mm. Group III: height 4 and diameter 2 mm. Group IV: height and diameter 4 mm. All teeth were then restored with the same procedure. The specimens were then placed in a thermocyclic machine. The microleakage was evaluated using 50% weight silver nitrate findings were statishtically evaluated with kruscallwallis test considering significant level of 0. 05. Result: There was no significant difference between Gingival leakages in 4 groups (p = 0. 163). There was a significant difference between 4 groups in occlusal leakage and groups 1 with 2, 2 with 3 and 2 with 4 revealed differences significantly. (P=0. 019) Comparing gingival and occlusal microleakage between groups 1, 3, 4.: there was not significant difference. (P>0. 05) Group 2, there was significant difference between occlusal and gingival microleakage at p = 0. 004 and gingival leakage rate was significantly greater than occlusal leakage. (P<0. 05) Conclusion: There was no significant direct correlation between C-factor and microleakage, but it seems, microleakage was increased with increasing the cavity dimention.