During the pea k perio d of it s repr oduct ion, e xter nal mo rphy log ical f eatur es o f go nad w ere under careful o bserv at ion for diploid a nd t riplo id oy ster, Ostr ea cucullata, and w ith a compar iso n of mature o ocy tes and their nuclei in size. The r esult s revealed that go nad in t riplo ids showed poo r develo pment re lat ive to diploids by appear ances, the diameter o f mature oo cytes a nd the ir nuclei of t riplo ids w ere 19. 6% and 17. 6% larger than tho se o f diplo ids, and w ith 70. 3% and 64. 2% larger in vo lume. Histo lo gical sectio ns pr esented that unlike diplo ids, gonadal development w as retarded in t riplo ids, of which the majo rity remained in sta ges of pr olifer at ion and spent , and only some of them dev eloped to growth and mature stages and produce mature o ocy tes and sperm. Three hermaphro dites wer e o bserv ed in t riplo ids sampled, w hich the sex ra tio w as female 7: ma le 1. In addit io n, retardat ion o f go na dal develo pment and hermaphro dit ic phenome no n of tr iplo ids are discussed in this paper .