Introduction: Fiber Reinforced Composite (FRC) post is one of choice in post root canal treatment. Resin cement polymerization results in shrinkage, creating micro gaps between the FRC posts and the root canal wall. This study aimed to find microleakage differences of prefabricated FRC posts with different root canal diameters. Methods: Twenty four maxillary central incisors were used, the root length was 13 mm from the apex. The root canal was prepared with conventional technique and then filled with a single cone technique. Samples were divided into 4 treatment groups with diameter preparation of 1.2 mm, 1.4 mm, 1.6 mm and 1.8 mm, cemented, then coated with sticky wax and nail polish and immersed in 2% methylene blue liquid and put into an incubator at 37o for 72 hours. The objects were divided into two mesiodistal directions and microleakage was observed under a stereomicroscope with 20x magnification. Results: Kruskal Wallis test showed there were significant differences in all treatment groups (p <0.05). Mann Whitney test showed differences between groups (p<0.05) of Ø 1.2 mm and Ø 1.6 mm group, Ø 1.2 mm and Ø 1.8 mm group, either of Ø 1.4 mm and Ø 1.8 mm group. Conclusion: This study concluded that there were microleakage in the four groups of post diameter, the wider root canal might lead to greater microleakage.