This investigation was carried out to study the effect of heat treatment at 170^oC and 200^oC at different times (0, 15, 45 and 90 min.) and different types of fried foods (fish, Taamia and Potatoes) on stability of some properties of cottonseed oil before and after thermal processing and identification of cis and trans ethylenic double bonds.  The results indicated the following points:- Refractive index increased in oil used for frying fish than both Potatoes and Taamia at 170^oC for 90 min., while at 200^oC for 90 min. it was higher in fried Potatoes oil than Taamia and fish samples frying oil. Acidity and acid value of deep fried fish oil increased at 170^oC for 90 min. than both frying oil of Taamia and Potatoes samples.  But at 200^oC for 90 min. data revealed that increase of frying oil of Taamia than both Potatoes and Fish samples oil frying. Decreasing iodine value for Fish frying oil than both frying oil of Potatoes and Taamia at 170^oC for 90 min.  While, data indicated that increase of iodine number at 200^oC for 90 min. for Taamia than fish deep frying oil, contrast iodine value decrease at 200^oC for 90 min. for Potatoes deep frying oil, due to natural chemical composition. Increased in peroxide value in frying oil of Taamia and Potatoes samples, while peroxide value decreased in oil deep frying of fish samples at 170^oC for 90 min.  On the other hand data revealed increased in peroxide value in deep frying oil of Taamia than of frying oil of Potatoes and Fish samples at 200^oC for 90 min. The red and yellow color increased at 170^oC for 90 min. for deep frying oil of Taamia than Potatoes frying oil, while data revealed that darkening of color for oil frying of fish samples.  At 200^oC for 90 min. red and yellow color was 4.75 and 35 respectively for fried oil of Potatoes, but darkening color for both Fish and Taamia and difficult determination. Using IR and UV instruments showed that trans ethylenic double bonds (carcinogenic) formed at 200^oC for 90 min. but disappeared at 170^oC at 90 min. for all fried oil samples.  The amount of trans-ethylenic double bonds was higher in fried fish, Taamia and Potatoes respectively.  The results indicated that successive frying of oil cause formation of carcinogenic and harmful substances to human health for all kinds of foods, especially fish samples, Taamia and Potatoes respectively, for 90 min. at 200^oC compared with control samples.  The most effect for producing such substances had been due to heat degree and frying periods