Flyingfish has significant meaning on three different dimensions: ecology, culture and fisheries at Kuroshio off Taiwan. However, the figures of catch dropped nearly 60% in 2006-2007, which has led scientist to pay more attention on the sustainability of resources for the species. On the other hand, with little knowledge of reproductive biology and life history of the species, it is difficult to change the situation. Reproductive biology is an important basic study, which is to discover the life history of a species ,also it assesses changes in their resources and reproductive potential. Cypselurus poecilopterus is the most important dominant species in Kuroshio waters of Taiwan, and also an economical species. The reproductive biological study of Cypselurus poecilopterus would be great benefit on further studies. This study performed from April 2008 to June 2010, sampling from diverse areas, Pingtung, Taitung, Hualien, Ilan and Ludao, collected total 2174 individuals, including 1756 males, 418 females and 7 were unable to identify. Fork length (FL) range from 15.2cm ~ 22.1cm. All samples were measured by the total length (TL), fork length (FL), body weight (BW) and other data, and the gonadal histological examination, and the female sex ratio, gonadosomatic index (GSI), condition factor (CF). The results shows that only 5% of female with fork length less than 18.5cm, while more than 80% with more than 20.5cm; via gonadal histological examination, the occyte development can be divided into chromatin-nucleolus stage, peri- nucleolus stage, yolk stage, migratory nucleus stage and ripe egg stage, five stages. When female GSI is greater than 4, it reaches matured, and the fork length of the smallest matured individual is 16.3cm, average length at 50% sexual maturity was estimated to be 18.6 cm FL for female; the annual reproductive period is April to August, and the monthly female sex ratio seems like would be raise with the closing of reproductive period. Fecundity is about 13000 ~ 36000; Cypselurus poecilopterus is estimated to be simultaneous mature species by the gonadal histological examination and egg diameter measurements.