Thermoplastic Polyoxymethylene (Polyacetal / POM) is one of the engineering plastics. It is very important for POM and its FRP to grasp accurately the viscoelastic behavior. Furthermore, POM is a crystalline polymer with very high crystallinity. Therefore, it is necessary to consider the effect of crystallization. The purpose of this study was to make clear the effect of crystallization on bending creep behavior of POM and glass fiber reinforced POM (GFRPOM). It is considered that there are two factors for polymer crystallization. One is crystallinity which is calculated by ratio of crystal and amorphous part, the other is crystal grain size which represents interior morphology. The creep test was performed using the materials which adjusted crystal grain size and crystallinity. As a result, these materials conformed to Arrhenius type of time-temperature superposition principle, and the effect of crystallinity on creep behavior had the time retardation effect. The effect of crystal grain size made the shape of master curves different every crystal grain size. Therefore, the different crystal grain size makes the different creep behavior. Only considering the effect of same crystal grain size, it was able to estimate the long term creep behavior on materials of arbitrary time, temperature and crystallinity.