Abstract: Based on the data collected in the west-central Indian Ocean during September 2005 to December 2005 and December 2003 to May 2004, the paper analyzed feeding behaviors of bigeye tuna Thunnus obesus preliminarily. The results indicated that diet composition of Thunnus obesus included Loligo, Sardina, Scomber, Penaeus, Portunus trituberculatus, Alepisaurus, Brama, Gempylus, Sepia officinalis, Balistes, Sparus, Physalia, Trachurus, and other species of fishes, in which Loligo (IRI=50.84%), Scomber (IRI=17.70%) and Penaeus (IRI=9.49%) are the main diets. The percentage of empty stomachs of T. obesus is 52.2% and 47.7% respectively to the male and female. Monthly fluctuation occurred in the diet compositions. Loligo pealei was dominant in the diet of T. obesus monthly, followed by Scomber japonicus，Sardina Antipa, Penaeus, and Portunus trituberculatus. Significant monthly fluctuations occurred in mean weight.