Aquatic Resource Science. Feeding ecology of Thunnus obesus in the western Atlantic Ocean. Observation on feeding ecology was conducted based on the data of Thunnus obesus captured by tuna longline fishery in the western Atlantic Ocean from August, 2004 to March, 2005. The food items in stomach content of Thunnus obesus consist of Sardina pilchardus, Loligo pealei, Penaeus japonicus, Brama brama, Alepisaurus ferox , etc. However, these have a significant monthly variation, Sardina pilchardus is the majority of prey (mean= 55.77%) in each month and followed by Loligo pealei. The significant monthly variation of mean stomach fullness and percentage of Thunnus obesus with empty stomach have been observed. In the group of their fork lengths (FLs) less than 100 cm, the percentage of empty stomach is decreasing with the FL increasing; when their FLs fall into 100-150 cm, the percentage of empty stomach is increasing sluggishly; and when their FLs greater than 150 cm, the percentage of empty stomach shows an insignificant variation, mean stomach fullness has a decreasing trend with the FL increasing. Mean stomach fullness has a decreasing trend basically with the increasing of FLs, and monthly mean stomach fullness varies unremarkably and mostly remain to 0.35-0.40. Percentage weight of Sardina pilchardus in major prey groups of each FL class of Thunnus obesus is kept on the level of 50% and more. Monthly Shannon-Weiner diversity index H′of Thunnus obesus has an insignificant variation and maintains on the index H′ level of 1.6. The index H' is quite stable at the FL in 90-160 cm, and decreasing if the FL is longer than 160 cm. 22 Refs.In Chinese. Key words:western Atlantic Ocean; Thunnus obesus; feeding ecology; fork length; Shannon-Weiner diversity index H′ ZHU Guo-ping(College of Marine Science and Technology, Shanghai Fisheries University, Shanghai 200090，China), ZHOU Ying-qi, XU Liu-xiong, JIANG Wen-xin//J Fish China,2007,31(1).23-30