A new spectrophotometric method was estalished for the determination of oleic acid. This method is based on the 1,2-dichloroethane extraction of the ion-pair formed between methylene blue and oleic acid. But the absorbance of an ion-pair in the 1,2-dichloroethane layer was variable with the temperature, the ion-pair was extracted back into a diluted hydrochloric acid solution. The maximum absorbance of the acid extract observed at 660mμ and a linear relationship was observed from the initial amount over the range of 50-800γ/㎖ of oleic acid in the aqueous phase. The composition ratio of the ion-pair formed between methylene blue and oleic acid was determined to be 2 : 1 by both the mole ratio and continuous variation methods.