A method for identifying and estimating the mixing ratio of soybean oil in an adulterated sesame oil was established using ten sesame oils and nine soybean oils. Fatty acid pattern similarity (cos θ) between sesame oil mixed with soybean oil and pure sesame oil was assessed and expressed : [(1-cos θ) 106] 1/2, as the value of 21/2 sin θ/2 × 103 which was proportional to the mixing ratio of soybean oil. cos θ between one sesame oil and nine soybean oils was calculated before determining the highest and lowest values of 21/2 sin θ/2 × 103. cos 0 between the average fatty acid pattern of ten sesame oils and each sesame oil was also determined. When the fatty acid pattern of sesame oil was specified or the average fatty acid pattern of sesame oil could be used, the mixing ratio of soybean oil was estimated from 21/2 sin θ/2 × 103. The percentage of linolenic acid was useful, however unsaponifiable matter was of no use for the identification of soybean oil.