As one of the major preys of many important economic fish species such as swordfish and dolphinfish in waters off estern Taiwan, flyingfish belongs to low-end consumers in the food chain with the function of maintaining the stability of the Kuroshio marine ecosystem. Hirundichthys oxycephalus is the primary component of flyingfish-egg fishery captures in the northeastern waters of Taiwan, and is also one of the dominant species of flyingfish in eastern waters of Taiwan. However, the significant drop of the flyingfish and flyingfish-egg catch from 2006 to 2007 and the effects on ecosystem and fishery caused major concern from the fishery sector and academic field. In order to manage this marine resource effectively, the phylogenetic relationships and population structure needed to be characterized first. In this study, the phylogenetic relationships of Hirundichthys oxycephalus of Northwestern Pacific was characterized based on the mitochondrial COI fragment. Totally 55 samples were collected between July, 2008 and November, 2010 in waters of Keelung, Ilan, Hualian, and Green Island. In addition, 12 more samples were obtained in Sebtember, 2009 from Tanegashima Island, and Yakushima Island of Japan. The DNA sequencing results of samples from Taiwan showed a total number of 29 haplotypes. The length of partial COI sequence was found to be 657 bp while the mean genetic distance was found to be 0.6%. In phylogenetic analyses, two major groups were identified in the phylogenetic trees by neighbor-joining and maximum-likelihood methods. The majority of "Keelung inshore group" came from Keelung and Ilan waters. The main population of "Kuroshio group" came from Green Island. The variation between two groups was found to be 61.75% by amova. The DNA sequencing results of samples from Japan showed a total number of 8 haplotypes. The length of partial COI sequences was found to be 657 bp with a mean genetic distance of 0.53%. In the phylogenetic tree, the samples from Japan were found to belong to "Kuroshio group". The variation between the two major groups was found to be 60% by amova. It was inferred that the differentiation of flyingfish into the two major groups in Taiwan was due to the flow pattern difference of Kuroshio in northeast waters of Taiwan. It was also inferred that phylogenetic similarity of the samples from Japan and the Kuroshio group was due to the distribution of both groups locating on the same path of the main current of Kuroshio. However, applying different distribution assumption may result in different conclusion such as one single stock hypothesis. Further studies will be needed to confirm the stock structure of the species.