The critical velocity of runaway electrons in a magnetized plasma is investigated from a kinetic theoretical approach. The results of numerical calculation are obtained for the parameter regions of tokamaks. The frictional force along the magnetic field encountered by the higher velocity runaway electrons is greater than that in a field free plasma. This means a higher critical velocity in a magnetized plasma. For an electron with large longitudinal velocity, the transverse velocity enhances the dynamic friction. This gives rise to the results of a higher critical velocity for the electrons with larger transverse velocity.