In this study, the cis to trans isomerization of unsaturated fatty acids were investigated about Soybean oil (SBO), Corn germ oil (CGO), Cottonseed oil (CSO), Margarine (MG), Shortening (ST) incubated at 40±2℃ for 35 days and heated at 185±2℃ for 24 hours. The contents and kinds of trans fatty acids in each sample were determined by GLC after seperating by HPLC. The results were obtained as follows; 1. When samples were incubated at 40±2℃ for 35 days, the contents of total trans fatty acids were increased from 0.5∼12.3% to 0.8∼20.5%. The kinds of trans fatty acids found were t-C_(18:1), t,c-C_(18:2), t,t-C_(18:2), t,c,c-C_(18:3) in SBo, t-C_(18:1), t,c-C_(I8:2) in CGO, t-C_(16:1), t-C_(18:1), t,c-C_(I8:2), t,t-C_(18:2) In CSO. Processed oil such as MG and ST showed more complicated composition of trans fatty acids than SBO, CGO and CSO. T-C_(18:1), t,c-C_(18:2), t,t-C_(18:2), t,c,t-C_(18:3) t-C_(20:1) were detected in ST. 2. In the case of heating at 185±2℃ for 24 hours, the contents of total trans fatty acid were 1.6∼37. 2% in all samples. Heating made more remarkable isomerization than incubation (40±2℃). Specially, c,c,t-C_(18:3) in SBO, t,t,t-C_(18:3) in MG, t,c,t-C_(18:3), t,t,t-C_(18:3) in ST were detected. 3. The total contents of trans fatty acids of processed oil were higher than vegetable oils. During incubation, trans fatty acids increased in the order of MG>ST>CGO>SBO>CSO, and during heat treatment, MG>ST>SBO>CGO>CSO.