|A total of 152 gonad samples of southern bluefin tuna were collected during April to September in year of 2010-2013. The fork length of samples concentrated between 100 and 135 cm. Based on the monthly trend of gonado-somatic indices, southern bluefin tuna might be reproductively active before April and May. This study also observed development of gonad and determined the maturity based on the histological sections of gonad samples. The results indicated that most samples were immature and few samples were mature with reproductively inactive. In addition, more samples were under regressed and regenerating stages, and thus mature fishes might migrate to sampling area after spawning during April and May. Gonad weights of most female and male samples were less than the gonad weight of smallest mature fish observed by Chen et al. (2013), while the gonado-somatic indices of fewer female samples were higher than the criterion of Chen et al. (2013). The gonad development for samples of this study is determining based on histological examination and the sexual maturity of these samples will be accurately provided. The estimated body length at 50% maturity was 144.16cm. However, this estimate may be highly uncertain due to insufficient mature samples.