It is important to understand the contribution of the ratio of in-plane and out-of-plane properties in order to clarify the flexural deformation of out-of-plane anisotropic material like CFRTP (carbon fiber reinforced thermoplastics) in detail. Since, in specifically, E1/G13 of CFRTP would be very large, in case that CFRTP is applied vehicle structure, it becomes important to know the influence of out-of-plane properties on structural deformation behavior. From the point of reliability design, additionally, the information of the time-dependence of flexural behavior is greatly helpful to analyze and design some kinds of structural parts. In this study, a theoretical model on time dependent flexural elastic modulus of CFRTP was tried to construct by time-temperature superposition principle on viscoelastic properties of matrix resin. The theoretical model was verified by dynamic three point bending test using the polypropylene-based discontinuous CF mat reinforced thermoplastics (CMT) with in-plane isotropic. As a result, it was quantitatively clarified that the time dependence of flexural stiffness of CMT is due to viscoelastic property of matrix resin. And the contribution of out-of-plane shear modulus to flexural modulus was also clarified quantitatively. These results indicate that the time and temperature dependence of flexural modulus of CMT is able to be predicted by Young’s modulus and out-of-plane shear modulus influenced from viscoelastic property of just matrix resin, volume fraction of fiber and coefficient configuration of reinforced-fiber. It is also clarified that out-of-plane properties affect flexural stiffness strongly at high temperature or low strain rate condition.