Casein is known as a source of bioactive peptides. In this study, the effects of mayonnaise enrichment with casein hydrolyzates on the oxidation process during 6 months were investigated. For this purpose, mayonnaise (≈60% oil) was treated with casein hydrolyzates and encapsulated casein hydrolyzates at concentrations of 100 and 200 ppm and compared with synthetic antioxidants (TBHQ). The results showed that the addition of casein hydrolyzates decreased the oxidation (peroxide, thiobarbituric acid, anisidine, and Totox indices) with a positive dose-response from 100 to 200 ppm. At similar concentrations, samples containing encapsulated casein hydrolyzates performed better than nonencapsulated casein hydrolyzates in retarding the oxidation and were able to compete with TBHQ. The results of scanning electron microscopy (SEM) also showed an improvement in the structure of samples containing encapsulated casein hydrolyzates. Therefore, the use of encapsulated casein hydrolyzates as natural antioxidants is recommended in functional foods and food emulsions such as mayonnaise.