The respo nses of insensi tiv e v ariety Kefeng 6 and sensitiv e one Naiyin Heido u ofsoybean to shor t day t rea tment a t V1- V12 dev elopmental stages w ere studied. Th e results show ed that the ef fect o n accelerating flo rescence w as max imum w hen short dayt reatment began a t V1 stage. With the delay of t rea tment , the effect of hastening flo rescencew eakened g radually, at the same time, that o f hastening ma turatio n w as enha ncedand then low ered sligh tly. The ef fect of t rea tment at V8 to V10 stag es o n hastening post- flow ering matura tio n w as st ro ng er than that at o ther stag es.