Background: Although resin composites are now the material of choice for most restorations, polymerization shrinkage remains a major deficiency and key shortcoming that complicate the use of such versatile restorative materials. Objective: The current study was designed to investigate the efficacy of fiber insert application on the cuspal deflection and microleakage of (MOD) cavities of bicuspids restored with resin composite restorations. Material and Methods: A total of 120 upper premolar teeth were selected and used in the present study. Each tooth was vertically embedded into self-curing acrylic resin where, the CEJ was kept 2 mm above the level of acrylic resin to mimic the alveolar bone support in healthy tooth. Forty premolars were used for cuspal deflection measurement and one hundred and twenty for microleakage assessment. Results: Cuspal deflection was significantly decreased of tooth restored with Ceram.X (0.0085±0.0031) using fiber inserts than that restored with Ceram.X resin composite only (0.0145±0.009) at (P= 0.032). Fiber inserts significantly decreased the cuspal deflection of tooth restored with Z350XT resin composite (0.0076±0.0015) than that restored without fiber inserts for the same material (0.0122±0.0051) at (P= 0.002). Conclusion: It could be concluded that the problem of cuspal deflection and microleakage in complex cavities prepared in premolars can be greatly reduced using fiber reinforced composite inserts. Nano filled composite with or without fiber insert has a better performance in cuspal deflection and microleakage than nanohyprid composite.