The present study was carried out to investigate the effect of preheating resin composite that cured at different curing times on its degree of conversion and also the flexure strength. A total of 180 resin composite specimens were prepared. They were divided into 3 main groups of 60 specimens each according to the pre-curing temperature used either 5o C (cooled in the refrigerator), 37o C or 54o C .Each group was subdivided into 3 subgroups of 20 specimens each according to the curing time used for 10, 20 or 40 seconds. 10 specimens used for degree of conversion test and the other 10 for the flexure strength test. A sensitive electronic balance was used in weighting an amount of 2 mg of uncured composite that was carried with a Teflon tweezers and placed in a porcelain mortar with 0.198 gm of potassium bromide powder, which gives neutral spectra, to be ground together and held under a hydraulic pressure device to form and shape a composite disc specimen with standardized dimensions. The composite disc was placed into an attachment in the optical compartment of the FTIR apparatus to test the uncured specimen. The spectra of the uncured sample were scanned in the transmission mode in infra red spectrum from 4000-400 cm-1. The steel mold with the composite disc inside was placed either in the oven, adjusted to the previously planned temperature either 37o C or 54o C, or in a refrigerator at 5o C. The disc was then light cured immediately after gaining the wanted temperature with a light curing unit for either 10, 20 or 40 seconds. The steel mold with the cured disc was then replaced again into the FTIR apparatus. For flexure strength test, rectangular bar specimens with dimensions of 25mm length, 2mm width and 2mm thickness were prepared. The compule was placed either in the oven adjusted to the previously planned temperature, either 37o C, 54o C, or placed in a refrigerator at a temperature of 5o C. The composite was injected directly from the compule into the mold, and then light cured for 10, 20 or 40 seconds. After curing the flexure strength was measured with a universal testing machine using the four point flexure strength test. Regression model with Two-way ANOVA was used in testing the significance of the effect of temperature, curing time and their interactions on the degree of conversion and flexure strength. ANOVA test showed that there was a statistically significant difference between the different interactions. 40 seconds / 54º C showed the highest significant degree of conversion, while with both 10 seconds / 5º C and 10 seconds / 37º C resin composite showed the lowest degree of conversion. ANOVA test also showed that there was a statistically significant difference between the different interactions affecting flexure strength. Tukey’s test showed that 40 seconds / 54º C had the highest flexure strength, while 10 seconds / 5º C and 10 seconds / 37º C showed the lowest flexure strength. Under the conditions of this study the following conclusions could be drawn:1.Pre-heating resin composite prior to photo-activation provides greater conversion requiring reduced light exposure than with room-temperature composite.2.Increasing polymerization temperature compensates the decrease in curing energy when using shorter curing times for resin composites.3.Flexural strength test provides a good predictor for the degree of monomer conversion.4.Flexural strength positivity affected by increasing Pre-heated resin composite temperature.Recommendations: 1.Preheating cooled resin composites is recommended to ensure an adequate degree of conversion. 2.Further investigations should be carried out to test the effect of preheating resin composites on other mechanical, physical or biological properties.