Semiconductor lasers are very different from other lasers because refraction variation can't be avoided when the gain is changed. Refraction variation can be introduced the theory of semiconductor laser by a dimensional parameter. This parameter is called linewidth enhancement factor (LEF). The value of LEF is very important for many aspects of laser behavior. The LEF characterizes the linewidth broadening and chirp due to fluctuation in the carrier density. A simple method to measure the linewidth enhancement factor of laser diodes is presented in this paper. The method uses the self-mixing effect at a weak feedback level. An optical beam is reflected and injected into the laser diode cavity by an external target, and is then mixed with the light inside the cavity, causing variations of the optical output power. The waveform of the optical power is determined by the feedback factor C and the LEF. A theoretical formula to compute LEF is proposed for the case when the feedback level C is smaller than 1. The experimental results show this method is feasible and simple when a laser diode operates at single longitudinal mode.