INTRODUCTION: advancements in composite restorations have been suggested, including preheating of composite resins before photo polymerization by a heating device. OBJECTIVE: The aim of the study was to investigate the effect of preheating before curing on microleakage and microhardness of composite resins. MATERIALS AND METHODS: thirty extracted human molars were used for microleakage assessment. Sixty class V cavities were made in the buccal and lingual surfaces of teeth, and were randomly divided into three groups according to temperature of composite (Group I: 10 °C, Group II: 24°C, and Group III: 54-60°C). Each group is further divided into two subgroups according to the type of composite (nanohybrid composite (Grandio Voco Germany), and microhybrid composite (Z250 3M Espe), where subgroups (IA, IIC and IIIE received nanohybrid composite), and subgroups (IB, IID and IIIF received microhybrid composite). Microleakage was determined by dye penetration test with scoring criteria from (0-3).sixty composite discs were used in vickers microhardness test (VHN),with same experimental grouping as used in microleakage assessment. VHN was determined using calibrated Vickers indenter. Data were collected, tabulated and statistically analyzed by using ANOVA F-test and Chi-square test (p<0.05). RESULTS: concerning microleakage ; there was no significant difference between the groups. The cervical margins showed more microleakage than the occlusal margins. The least microleakage was obtained in subgroup IIIE with mean 0.5 ± 1.08 in 80% of specimens occlusaly and, also IIIE showed the least microleakage in 50% of cases with mean 1 ± 1.25 cervically. Concerning top microhardness; subgroups IA obtained the highest mean top VHN 113.35 ±10.1 followed by subgroup IIIE 108.43 ±1.52.While regarding bottom microhardness subgroup IIC obtained the highest mean bottom VHN 88.49± 1.29, followed by subgroup IA with mean value85.49 ± 9.69. CONCLUSION: it was concluded that preheating enhanced microhardness of composite resin materials, but have no effect on microleakage.