INTRODUCTION:Bulk-fill composites are promising restorative materials which may minimize placement time yet provide successful, durable esthetic restoration. OBJECTIVES: The purpose of this in vitro study was to evaluate marginal microleakage of nano-hybrid bulk-fill composite, and compare it to incrementally placed conventional nano-hybrid composite when used to restore proximal cavities in primary molars. MATERIALS AND METHODS:The sample consisted of 44 freshly extracted sound primary molars. Standardized proximal cavity preparations were prepared and the sample was divided randomly into 2 equal groups: Group I (n=22): Teeth were restored with nano-hybrid bulk-fill composite, Group II (n=22): Teeth were restored with incrementally placed conventional nano-hybrid composite. All specimens were thermocycled and immersed in 2% methylene blue dye at 37C˚ for 24 hours. Methylene blue penetration was assessed on a 4 point grading scale (0 to 3) using a stereomicroscope to evaluate the microleakage of the restorations. Data were statistically analyzed using Mann-Whitney test and Wilcoxon Signed Ranks test. RESULTS: No statistically significant difference has been found in the microleakage scores of group I at occlusal and cervical margin (p=1.000). In group II, no statistically significant difference has been found in the microleakage scores between occlusal and cervical margins (P=0.366). Comparing the microleakage scores of group I and group II, no statistically significant difference has been found (p=0.240). Although no significant difference was detected, bulk-fill composite showed lower microleakage scores than incrementally placed conventional composite. CONCLUSIONS: Bulk-fill composite restoration marginal integrity was comparable to incrementally placed conventional composite in proximal cavities of primary molars.