The relationships between muscle fat content and biological parameters of the yellowfin tuna (Thunnus albacares) were studied to determine if there were any relationships between them and to better understand its characteristics of growth, reproduction and the fishing ground. The biological data and muscle fat content of 91 yellowfin tuna were collected from the longline fishery in waters of 6°33'N~10°33'S, 44°54'E~68°32'E, Western Central Indian Ocean from October, 2013 to April, 2014. Histogram count figures were obtained to show the spatial or temporal distribution of fat content and fat content by gender, dressed weight, and gonad maturity. A generalized additive model (GAM) was used to analyze the relationships between fat content and condition factor (K), somatic index (SI), gonadosomatic index (GSI) and fork length (FL). Results showed that:① the fat content of yellowfin tuna was in the range of 0.1%–26.7%, and the average was 6.94%; ② the fat content of yellowfin tuna was low from October to December, 2013, and then was increasing after December, 2013. The fat content of (2°N~3°N, 59°E~60°E) was the highest (15.3%) and that of (7°S~8°S, 44°E~45°E) was the lowest (1.2%), and there was significant differences among them; ③ there was no significant difference among the fat contents by gender, gonad weight, gonad maturity stages or fork length; ④ By GAM, the results showed that there was no significant correlation between fat content and K, SI, GSI or FL. Results of this study suggest that:① the yellowfin tuna begin to reproduce in March and April; ② the area of 2°N~3°N, 59°E~60°E might be an important spawning ground; ③ there was no significant correlation between female reproductive capacity and muscle fat content.