The photodecomposition of herbicides may be affected by adding vegetable oils to the spray tank. In this study nine vegetable oils were compared to assess the photodecomposition of sethoxydim under natural light conditions. The experiment was conducted as completely randomized factorial design with three replicates at the College of Agriculture, Ferdowsi University of Mashhad, Iran, in 2013. Each herbicidal solution (with and without vegetable oil) was exposed to sunshine with time intervals of 0, 5, 10, 20, 30, 60, 120, and 240 min. The results revealed that the half-life value was increased by adding castor bean and cottonseed oils to 1.39- and 1.18-fold, respectively, compared to nonvegetable oil. These values for turnip, olive, corn, soybean, sunflower, canola, and sesame oils were decreased down to 4.74-, 2.38-, 1.81-, 1.75-, 1.52-, 1.28-, and 1.11-fold, respectively. A positive relationship existed between the half-life of sethoxydim in the presence of vegetable oils and their viscosity. However, a negative relationship was monitored between unsaturated/saturated fatty acids ratio and the monounsaturated value with half-life. A positive relationship also existed between saturated fatty acids, polyunsaturated fatty acids, palmitic acid, and linoleic acid with half-life. This study revealed that the amount of fatty acids in vegetable oils is a determining factor in preventing or facilitating the photodecomposition of sethoxydim.