Tunnel magnetoresistance (TMR) was investigated for fully epitaxial Fe/MgO/Co2MnSn magnetic tunnel junctions (MTJs) fabricated on MgO (001) substrates. The bias-voltage-dependence of the TMR ratio showed that the sign of TMR ratio at 300 K becomes negative for the bias voltage between −800 mV and +200 mV, which may be caused by the impurity state or other imperfections at the interface. The sign change in TMR effect was also observed with changing the temperature at low fixed bias voltage, implying that the effect is not simply due to a bias-dependent change of the density of state at the Fermi level.