Mitigative effects of phenolic compounds on lipid oxidation of tuna oil in gelatin film pouches from horse mackerel (Trachurus japonicus) scales were investigated during storage at 40 °C for 30 days under dark conditions. Improvements in the antioxidant activity of gelatin films were observed following the addition of phenolic compounds. Increases in peroxide values (PV) and 2-thiobarbituric acid reactive substance (TBARS) levels of tuna oil were significantly mitigated using gelatin pouches with ferulic acid, which had the fewest phenolic hydroxyl groups among the phenolic compounds tested. The ferulic-gelatin film also exhibited the highest DPPH radical-scavenging activity among the tested samples. These results indicate that the oxidation of lipids in tuna oil can be mitigated using gelatin films containing phenolic compounds, but its antioxidant activity is not directly related to the number of phenolic hydroxyl groups. Protein crosslinking in gelatin film is likely to be induced by aldehydes such as malondialdehyde (MDA), which is a final product of lipid oxidation, and this was observed following storage of tuna oil. Thus, it is suggested that care should be taken when using TBARS levels to evaluate lipid oxidation of foods in fish gelatin film pouches, since aldehydes including MDA are consumed for protein crosslinking.