Linoleic acid and conjugated linoleic acid were sampled in glass bottles charged with air and nitrogen re-spectively, and were irradiated with a)Co at different dose level. Peroxide value of samples were determined every two days. The results showed that the irradiation dose and storage period had great effect on peroxide value of two samples. Conjugated linoleic acid was more stable than linoleic acid during oxidation process. Relationship between the peroxide value and reaction mechanism under the y - irradiation was also discussed.