Introduction: The aim of this study was to in-vestigate the effect of irradiation duration and distance of light tip to prepared samples in two different light curing unites on the degree of conversion of resin composite. Materials and Methods: Conventional halogen and blue Light emitting diode (LED) units were used for polymerization of an Opalis resin com-posite. The conversion measurements were carried out at 0 and 8 mm distance from the directly irradiated surface in manufactured recommended curing time as well as in doubled curing time. The curing efficiency was observed with FTIR (Fourier transform infrared spectroscopy). Data was analyzed using ANOVA and Tukey post hoc test. Results: The LED curing unit exhibited the most homogenous in-depth curing efficiency at the same time (20 sec). The most amount of con-version degree in LED group was related to dis-tance 0 and irradiation time of 20 sec. However, in halogen groups the highest grade was observed in distance 0 and exposure time of 40sec. With increasing the distance between light guide tip and resin composite to 8 mm, being inevitable especially in class 2 box filling Conclusion: The result of our study confirmed that the best result for each group obtained in maximum contact with light source meanwhile reduplicating the manufacture recommended exposure time. Besides, in our research, LED showed a better outcome in proposed time of 20 sec in both distances.