Abstract: Phase locking of a diode laser array by an external cavity consisting of a mode selecting mirror or a phase grid was studied theoretically, on the basis of Fresnel diffraction and coupled mode theory. The relative complex amplitude distribution of the emitters operating in different super modes was computed for both cases. The compared results with sinusoid curves demonstrate that the mode selecting mirror is effective for in-phase mode selection and fluctuation elimination of amplitude resulted from edge effect, and the phase grid is also effective for in-phase mode selection.