The crude Silybum Marianum oil and sunflower oil were used for repeated deep fat frying of potato-fillet (French fries) for 5 consecutive days @ 20 minutes per day at 180-190oC and then allowed to cool for 18 hours at 25oC. The test oils were evaluated for physical and chemical deterioration. The quality indicates peroxide value (POV), anisidine value (AV), iodine value (IV), free fatty acid (FFA) and color (O.D at 420nm) of the samples were measured. It was found that all the quality parameters significantly increased as frying time was advanced, while the reverse trends regarding IV was observed in all the test oil. The highest average POV (38.1meq/kg), FFA (15.9%) and AV (9) were observed in Silybum Marianum oil while IV (97.1g/100g) in sunflower oil. The maximum change in color (discoloration) was observed in sunflower oil.