The object of this study was to investigate the immune-enhancing effects of Chlorella vulgaris (CV) on a deteriorated immune function by a protein-energy malnutrition (PEM) diet. Unicellular algae, CV were used as a biological response modifier. Male C57BL/6J mice were fed for 15 days with standard diet or a PEM diet, which is associated with decreased host immune defense. After 8 days, mice in the PEM diet group were orally administered by 0.05, 0.1, and 0.15 g/kg body weight of CV or distilled water. Nutritional parameters, and interferon (IFN)-$gamma$ levels were significantly increased in the blood serum of the CV (0.15 g/kg)-treated group (29.6$pm$2.8 pg/mL) compared to the non-treated PEM group (4.1$pm$0.4 pg/mL, p<0.05). In addition, cell proliferation and production of cytokines were investigated via a CV (0.01, 0.1, and 1 mg/mL) treatment using a human T cell line MOLT-4 cell. The CV treatment (1 mg/mL) significantly increased the production of both IFN-$gamma$ and interleukin (IL)-2 (51.3$pm$3.4 and 285.9$pm$18.8 pg/mL, respectively) compared to the control (51.3$pm$3.4 and 442.6$pm$14.3 pg/mL, respectively), but did not affect the production of IL-4. These results suggest that CV may be useful in improving the immune function.