Objectives: This in-vitro study was designed to investigate the effect of preheating two bulk-fill resin composites on the polymerization shrinkage strain. Materials and Methods: A total of 30 resin-based composite discs were prepared and divided into two equal groups: a thermo-viscous one (VisCalor Bulk) and a high viscosity resin-based composite (X-tra fil). For VisCalor composite resin specimens a VisCalor Dispenser (VOCO, Cuxhaven, Germany) was used for pre-heating of the composite specimens to be injected into a Teflon mold. A Teflon mold with dimensions 7×4×4mm was fabricated to produce the standardized resin composite specimens. The reason for selecting a Teflon frame was to prevent adhering of the resin composite to the frame and so allowing its free shrinkage. A foil electrical resistance strain gauge was used to measure the polymerization shrinkage strain of the resin composite specimens. Analyzing the results was conducted using SPSS version-20, set at Significance level p≤0.05. Then, data was recorded, tabulated, and statistically analyzed using Kolmogorov-Smirnov and Shapiro-Wilk tests. Results: X-tra fil group has a significantly higher value (-1130.16 ± 237.56) in comparison to VisCalor group (-616.13 ± 114.75). The difference between the groups was (514.02 ± 12.17) and was statistically significant (p = 0.000). Conclusion: Preheating of VisCalor bulk-fill resin composite to 65⁰C has a superior effect to a degree on decreasing the polymerization shrinkage strain than the high viscosity bulk-fill resin composite.