The aims of this study are to investigate the effect of light intensity, curing time and combined effect of light intensity and curing time on the degree of conversion, depth of cure and amount of residual monomer released from composite samples cured with different light curing units. Single light cured composite restorative material (Herculite XR) exposed to different types of light curing units (Optilight LD, ULTRA-LITE 200 E plus, Astralis-VIVADENT) at a different exposure times (20, 40 and 60 seconds). The degree of conversion of composite resin samples was investigated by Fourier-transform infrared spectroscopy.The depth of cure of composite resin samples was investigated by scraping technique using a digital caliper. While the amount of residual monomer released from cured composite samples was investigated through measuring the released residual monomer in doubled distilled water with (CECIL 2000) ultraviolet visible spectrometer. The results it was shown that the low intensity LED LCU (Optilight LD) exhibit a non significant result in degree of conversion from those of high intensity (LED and QTH) LCUs at extended period of curing time.