The generation of oxidative stress, characterized by enhanced reactive oxygen species (ROS) formation, has been found in obesity. ROS production was increased during the differentiation of 3T3-L1 cells into adipocytes. We previously reported that caffeic acid phenethyl ester (CAPE) suppresses 3T3-L1 differentiation to adipocytes through the inhibition of peroxisome proliferator-activated receptor γ. In this study, the preventive effect of CAPE on oxidative stress in 3T3-L1 cells was observed. The results were as follows: (1) ROS production during 3T3-L1 cell differentiation to adipocytes was significantly (p < 0.05) suppressed by CAPE treatment in a concentration-dependent manner, (2) with CAPE treatment, the extracellular superoxide dismutase mRNA expression level significantly increased, but the NOX4 mRNA expression level did not change, and (3) CAPE treatment significantly increased superoxide dismutase (SOD) activity in 3T3-L1 cells. From these results, we suggest that the increased oxidative stress in 3T3-L1 differentiation to adipocytes is attenuated by CAPE treatment. This attenuation may be partly caused by increased SOD production.