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.