Using Lang's equation for the dynamics of injection locking of a laser diode, we show that the hysteresis property of the excess carrier density has direct influence on the mode-shift characteristics, which makes the shift of the slave laser mode to be different from the frequency detune of the external master laser signal within the locking range of the slave laser. The theoretical prediction is experimentally confirmed by the measurement of the mode-shift of an injection locked Fabry-Perot laser diode.