The effect of spin mixing due to thermal spin waves and the temperature dependence of the hot-electron spin polarization to the collector current in a spin-valve transistor (SVT) has been theoretically explored. We calculate the spin dependence of the collector current as well as the temperature dependence of the magnetocurrent at finite temperatures to investigate the relative importance of the spin mixing and the hot-electron spin polarization. In this study the inelastic scattering events in ferromagnetic layers have been taken into account to explore our interests. The theoretical calculations suggest that the temperature dependence of the hot-electron spin polarization makes a substantial contribution to the spin-dependent magnetotransport at finite temperatures in the SVT.