This study was performed to investigate the effect of olive oil diet on the immune response in ICR male mice. Experimental diets of 4 groups were fed ad libitum to the ICR male mice for 27 days. The results of this study were summarized as followings: 1. 10% Olive oil diet group as compared with the control diet group significantly decreased liver weight rate but significantly increased hemagglutination titer (HA), Arthus reaction, delayed type reaction (DTH), rosette forming cell (RFC), and phagocyte activity. 2. 20% Olive oil hypersensitivity diet group as compared with the control diet group significantly increased body weight gain, liver weight rate, and HA but significantly decreased Arthus reaction, DTH, RFC, phagocyte activity, and peripheral circulating white blood cell (WBC). 3. 30% Olive oil diet group as compared with the control diet group significantly increased liver weight rate but significantly decreased body weight gain, Arthus reaction, plaque forming cell (PFC), DTH, RFC, phagocyte activity, and WBC. The results showed that the increase of olive oil doses significantly decreased humoral and cellular immune responses, phagocyte activity, and WBC.