In this study, the antioxidative capacity of caffeic acid (CA), ascorbyl palmitate (AP), α-tocopherol (α-TO), and caffeic acid phenethyl ester (CAPE) was evaluated under the thermal oxidation model, in which 200 ppm of each compound was added to soybean oil, followed by thermal oxidation at 180°C for 32 h. Change of viscosity, acid value (AV), conjugated dienoic acid value (CDAV), p-anisidine value (p-AV), total polar materials (TPM), and the ratio of C18:2 to C16:0 (LA/PA) were evaluated during the reaction. All antioxidants showed significantly lower viscosity, TPM, and p-AV, and higher LA/PA, than the control (without antioxidant, CON), indicating that thermal oxidation was delayed. Among them, CAPE showed significantly lower viscosity, TPM, and p-AV, and higher LA/PA, than the other antioxidants (p < 0.05). In the correlation between the oxidation parameters measured from CON and CAPE, the correlation coefficient between p-AV and viscosity was rather low at r = 0.7603 (in CON) and r = 0.7338 (in CAPE), respectively.