The longtail tuna (Thunnus tonggol) is an economically important pelagic species inhabiting tropical to temperate provinces of the Indo-Pacific Ocean. As a result of their coastal distribution, longtail tuna are heavily exploited by small-scale commercial and artisanal fisheries in at least 17 countries throughout this area. They are mostly targeted by purse-seine, gillnet, and trolling fisheries. Thailand, Taiwan,Indonesia, Malaysia, and Iran contribute most to reported landings that exceed 100,000 t per year. As longtail tuna contribute to important artisanal and subsistence fisheries in many countries, these reported landings are likely to be significantly underestimated. In recent years,the international research for longtail tuna is more and more emphasis on the stock status. However, there were very few attempts have been made for longtail tuna in waters off northwestern Pacific Ocean. The objectives of this study are study the population characteristic and trophic level of longtail in waters off eastern Taiwan. Furthermore, theinternational cooperative research project shall be made between Taiwan and CSIRO Australia for exchanging samples and data combining. These results will provide important biological parameters for future stockassessment of longtail tuna.