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 $40{pm}2^{circ}C$ for 35 days and heated at $185{pm}2^{circ}C$ 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 $40{pm}2^{circ}C$ for 35 days, the contents of total trans fatty acids were increased from $0.5{sim}12.3%$ to $0.8{sim}20.5%$. 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 $SBO,;t-C_{18:1},;t,c-C_{18:2}$ in $CGO,;t-C_{16:1},;t-C_{18:1},;t,c-C_{18:2},;t,t-C_{18:2}$ 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 $185{pm}2^{circ}C$ for 24 hours, the contents of total trans fatty acid were $1.6{sim}37%$. 2% in all samples. Heating made more remarkable isomerization than incubation $(40{pm}2^{circ}C)$. 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.