The aim of this study was to determine the degree of conversion and residual stress of resin composite preheated to different temperatures and to compare it to room-temperature composite. The composite resin was preheated to 40 degrees C, 50 degrees C, and 60 degrees C and packed into brass rings and light-cured. The degree of conversion and residual stress were analysed using Fourier transform infra-red spectroscopy and X-ray diffraction, respectively. The results obtained were tabulated and statistically analyzed using Kruskal-Wallis test and Tukey's honestly significantly different test. The results showed significant increase in the degree of conversion and residual stress with increase in preheating temperature.