It is important to master the knowledge of the ovary development of skipjack,to provide a fundamental information for further study on reproduction mechanism,spawning date and times,batch fecundity,body size of first sex maturation etc.This is also the basic data for the skipjack biological study and resource assessment.140 ovaries of skipjack(Katsuwonus pelamis)have been studied by means of histological method,and the samples were collected from the EEZs of Papua New Guinea and Federal States of Micronesia in the Western and Central Pacific(WCPO)during October 2007 to January 2008 on board purse seine vessels.Samples for histological study were fixed in formalin solution and processed by the standard methods.Sections(thickness 7 μm)were stained by hematoxylin-eosin.In the course of studying ovogenesis,we observed ovary features through Olympus BX-51 microscope camera and determined ovarian development stages.Full stages of maturity and cells progress have been observed and described in detail in this paper reflecting the ovogenesis development.The main results are:(a) Mature development of oocyte can be marked as six periods.According to that,six phases are outlined for ovary.At the first stage,the cell is small and the ratio of nucleolus to cytoplasm is big.At the second stage,a follicular cells layer is around the nucleolus,cytoplasm becomes two layers,there are some small fat drops around the nucleolus.At the third stage,yolk granules occur and increase gradually.At the fourth stage,the cell volume grows bigger while fat drops fuse and yolk granules fuse.At the fifth stage,fertilization hole pouches and ovulation is happening.And at the sixth stage,nucleolus blurs and cytoplasm decreases.(b) Skipjack is a type of multiple spawning species,varied oocyte phases can be observed in one ovary at the same time.GSI changes obviously when ovary develops from stage Ⅲ to Ⅳ and from Ⅴ to Ⅵ,but there exists much overlap from Ⅰ to Ⅵ period.(c) Traditional gonad identification system shows obviou inaccuracy for skipjack tuna and for the gonad in Ⅵ period especially.For stagefigonad,identification by traditional system are almost consistent with histological system.The higher stages of ovarian development,the greater error between the two methods.When gonad is in stage Ⅰ,gender cannot be identifed by naked eye,so comparison only be made from Ⅱ to Ⅳ.(d) For average GSI of each day from Oct.22th 2007 to Jan.2nd 2008,there are two peaks during late November(early December),the lowest point of close to 2,up to 10,generally more than 4.Late December,GSI decreased between 0 and 2.The end of December(early January),another peak occurred,and gradually more than 3,and up to 4 or more,close to 6.Two peaks of spawning period have been identified during the surveys,and they are from the end of November to the early December,and from the end of December to the early January.