Yellowfin tuna (Thunnus albacores; YFT) is one of the three tropical tuna species in the world. It is also one of the main target species of China's distant water tuna fishery. Based on 229 female YFT samples collected by China's national tuna fishery observers in the tropical Pacific Ocean (14°1'S-11°41'N,158°43'E-128°24'W), the reproductive characteristics of YFT were analyzed using histological method. The majority of samples were observed in maturity stage Ⅰ and stage Ⅱ,and 36.7% of samples were mature. The proportion of mature individuals and fork length can be described as P=1/(1+e^{[-0.072(L_F-138.13)]}), with size at 50% maturity of 138.13 cm.In the southern hemisphere, the proportion of mature samples and gonadosomatic index increased with latitude. The maturity features differed inthe eastern Pacific Ocean (EPO) and the western andcentral Pacific Ocean (WCPO, divided by 150°W), in that the YFT in the EPO attained sexual maturity earlier than those in the WCPO. The seasonal spawning activity also differed between EPO and WCPO. The variations of gonadosomatic index by maturity stage showed similar pattern between EPO and WCPO. Our results also indicated that the visual observation tended to make higher errors in determining the maturity stages of I and IV; while make lower errors in determining the maturity stages of Ⅱ and Ⅴ.