In this study, the rut susceptibility of warm mix asphalt (WMA) was evaluated by laboratory material characterization tests that included repeated creep testing using universal testing machine (UTM-5P) on WMA and conventional hot mix asphalt (HMA) mixtures. With regard to the fact that aggregate makes up a high proportion of volume and mass of mixtures, this parameter is considered as an important constituent of asphalt concrete. So, one aggregate source, three gradations and three different percentages of Sasobit® content were used to manufacture mixtures. Four creep parameters, namely, flow number (FN), creep stiffness (CS), creep strain slope (CSS) and secondary initial point (SIP), were used to analyze the test results. The primary objective of this research effort was to conduct a comparison between permanent deformation of HMA and WMA mixtures and also to evaluate the effect of aggregate gradation and different Sasobit® contents on introduced creep parameters. Analysis of data shows that the creep behavior of both asphalt mixtures (HMA and WMA) was influenced by aggregate gradation and also by Sasobit® content. Comparison of test results between HMA and WMA exhibits that, in general, WMA mixtures exhibit improved rut resistance compared to conventional mixtures. Also, it is concluded that creep strength of WMA mixes are more susceptible to change in aggregate gradation compared to HMA mixes, while increasing in Sasobit® content produce more stable mixtures. In addition, it is determined that finer aggregate gradations were more influenced by change in the Sasobit® content.