This paper explains the reason for the difference in maximum torque-speed characteristics and power factor of IM between motor and generator modes and investigates the influence of machine design parameters on the difference, such as stator resistance, rotor resistance and iron loss etc. An analytical mathematical model based on the dq reference frame is employed for investigation. It is shown that the difference in torque-speed characteristics between motor and generator modes is mainly caused by the opposite direction of q-axis current and affected by stator resistance value. Finally, analytically calculated torque-speed characteristics of a 12kW IM in motor and generator modes are verified by finite element analyses.