Mostafa A. Abd El-hameed¹, Essam E. Al-Wakeel², Noha A. El-Wassefy^{3 1} Demonstrator, Department of Dental Biomaterials, B.D.S. , Faculty of Dentistry, Mansoura University. ²Professor of Dental Biomaterials, Faculty of Dentistry, Mansoura University. ³Associate Professor of Dental Biomaterials, Faculty of Dentistry, Mansoura University. Abstract: Objectives:The purpose of the present study was to evaluate a bulk fill resin composite comparing it with a conventional build up one, using two different curing lights, quartz tungsten halogen (QTH) and light emitting diode (LED), regarding; degree of conversion (DC), microleakage, microhardness and fracture toughness. Methods:Two nanohybrid resin composites were investigated in the study. One is a bulk fill (Tetric N-Ceram Bulk Fill) and the other one is placed incrementally (Tetric N-Ceram). Specimens of each resin composite were divided into two equal groups (group A cured with QTH and group B cured with LED). DC, microleakage, microhardness and fracture toughness of different groups were studied. The results were then statistically analyzed and compared. Results:There were no significant differences among the materials tested, regarding; DC, microleakage, microhardness and fracture toughness. In addition, none of the used curing lights had a pronounced effect on the properties of the tested composites. Conclusions:The tested properties of Tetric N-Ceram Bulk Fill composite resin were nearly equal to its counterpart incremental one. Both types of lights were nearly equally effective in curing both types of composite resin. Keywords:Bulk fill composite, Light curing units