Aim: The aim of this study was to evaluate and compare the mechanical properties of three resin composites core materials with different filler particle sizes. Materials and methods: nanofilled (Filtek Z350 XT , 3M, ESPE USA), microhybrid (Te Econom, Ivoclar, Vivadent, India) and nanohybrid (Nexcomp, Meta biomed, korea) resin composites were used during this study. Mechanical properties regarding the compressive strength, diametral tensile strength and surface hardness were tested and results were collected and evaluated. Data were statistically analyzed using one-way Analysis of Variance and Tukey HSD test. Results: The nanofilled type revealed the highest significant compressive and diametral tensile strength values while the nanohybrid type showed the least significant hardness values among the results of the two types of composite resin for the three tests. Conclusions: after evaluation it was shown that the tested nanofilled resin composite type is more appropriate for use as core material compared to the other tested types.