Abstract: From 1996 to 1997 , experiments were carried out to induce triploid in the Pacific oyster (Crassostrea gigas ) by inhibiting release of polar body I of the zygotes with 6- dimethylaminopurine (6-DMAP).L16(4 5)design was selected to engage in orthogonal experiment of three factors and four levels :the four levels of 6-DMAP concentration were 150 , 300 , 450 and 600 μmol/L , respectively ;the induced occasion (the time after sperms and eggs confused)was 10 , 15 , 20 and 25 min , respectively ;and the induced duration was 10 , 15 , 20 and 25 min , respectively . Experiments were repeated twice .The highest induced ratio was (71 .3 ±1 .2)%, the embryonic hatching ratio of this experiment group was (55 .5 +3 .1)%, and the abnormality of D-shape larvae was (10 .7 ±1 .6)%.The results calculated by direct analysis indicated that the best level combination of inducing factors for triploid in Pacific oyster was 25 -25 .5 ℃water temperature , at 10 min after sperms and eggs confused and the fertilized eggs were rinsed continuously in sea water with 600 μmol/ L 6-DMAP, the treatment duration is 15 min .The sequence of three factors for deciding triploid was : 6-DMAP concentration ※ induced occasion ※ induced duration .The influence of the factor 6-DMAP concentration on the triploid was very obvious , but the influences of the factors induced occasion and induced duration were not obvious .