The oil or fat composition of the invention contains a monoacylglycerol and/or a diacylglycerol in a total amount of 5-100 wt. % and which exhibits an index of stability against oxidation (induction time as measured through a Rancimat test conducted at 100° C.) of 7 or higher, wherein the monoacylglycerol and/or the diacylglycerol contain, as fatty acid constituents, omega3 unsaturated fatty acids in amounts of 15-90 wt. %. Through ingestion thereof in a small amount without drastically changing the persons lifestyle, the highly harmless oil/fat composition of the present invention provides effects for reducing and preventing accumulation of body fat, for reducing and preventing accumulation of visceral fat, and for prevention and treatment of obesity and exerts an excellent prevention and treatment effect on life-style related diseases through long-term ingestion. Thus, the composition of the present invention is useful for foods, pharmaceuticals and pet food or feed.
