Based on the catch data of yellowfin tuna captured by Chinese tuna longline fleet in the west-central Indian Ocean from January to June in 2003 and 2004, the basic biological characteristics of catch composition of yellowfin tuna captured were investigated. The result indicated that the processed body weight of yellowfin tuna captured varied between 7 - 81 kg （2003） and 20- 78 kg（2004） respectively, and the dominant processed body weight varied between 20- 50 kg （2003） and 30 - 60 kg （2004） respectively. The fork length of yellowfin tuna captured varied between 70 - 180 cm and 110 - 170 cm in 2003 and 2004 respectively. Mean sex ratio remained some 0.71 and 1.00 in all months in 2003 and 2004 respectively. Monthly sex ratio had an insignificant difference among all months in 2003, however, the ratio varied remarkably during 2004. To frequency of monthly feed fullness, grade 1 and 2 dominant in every month of 2003 and insignificant difference existed among all months. Grade 1 dominant in every month of 2004, and large differences existed in 2004. The sex ratio fluctuates remarkably at a LJFL of less than 100 cm. The variation on sex ratio is insignificant and maintaining to some 0.5 when LJFL falls into 100 - 150 cm. All samples are males at LJFLs larger than 160 cm. The processed body weight and fork length of yellowfin tuna captured had the relationship curves W = 2 ? 10^- 5 L2.9269 between them.