Frying process is one of the cooking's techniques usingvegetable oil. This process is commonly used in food industry, restaurants, food services, food retail and householdscale. This is a laboratory experimental study which performed in laboratory of Public Health Nutrition FKM-UI andIntegrated Laboratory IPB, Bogor from December 2005 until March 2006. It was conducted by two (2) type oftreatment (used cooking oil ex cassava and meat) with 4 (four) times for each treatment. The objective of this study is toknow the influence of frying by using deep frying (frying in high temperature and in a long time) and repeating to transfatty acid formation in cooking oil. From the result revealed that fatty acid type mostly contained in a fresh cooking oilis oleic acid. Trans fatty acid was formed after second repeating of deep frying and increased in line with the frequent ofrepeating. Correlation test result had shown that negative association between elaidic acid (trans) and oleic acid (cis)(r = - 0,8; p value = 0.016). In accordance with the beginning of trans fatty acid formation, it would be better to use thecooking oil not more than twice.