In this study, we described and analyzed the fisheries biology of blue shark based on the total of 336 samples collected by China's national tuna long-line fishery observers in the Indian Ocean (1°-33°N, 40°-75°W) from October 2012 to September 2014.The results indicated that the dominant fork length of males and females was 150-229.9 cm and 150-250 cm. The captured female-male ratio was near 1:1 and there were no significant differences between the mean fork lengths of male and female. The larger individuals were mainly distributed in the northern Madagascar waters (x=207.6 cm), while the smaller individuals were mainly captured around the southeast Madagascar waters (x=170.9 cm). The mature individuals of bycatch blue shark were much more dominant in northern Madagascar waters than in the Southeast Madagascar waters. Fork length at 50% maturity was estimated to be 183.6 cm for females and 182.3 cm for males. Squid and long-nose lancetfish had the highest occurrence in stomach contents of the blue shark. This study contributed to a better understanding of fisheries biology of the blue shark in the Indian Ocean and would be useful for evaluating the shark's resources.