Hizikia fusiforme is a commonly used food that possesses potent anti-bacterial, anti-fungal, and antiinflammatory activities. The immunostimulatory activities of aqueous extract of Hizikia fusiforme (HFAE) in RAW 264.7 macrophages and whole spleen cells were investigated. HFAE activated RAW 264.7 macrophages to produce cytokines such as nitric oxide (NO), tumor necrosis factor-${alpha}$ (TNF-${alpha}$), interleukin-$1{eta}$ (IL-$1{eta}$), and interleukin-6 (IL-6) in a dose-dependent manner. In addition, HFAE induced the mRNA expression of TNF-${alpha}$, IL-$1{eta}$, and IL-6 in RAW 264.7 macrophages. Moreover, HFAE stimulated proliferation of whole spleen cells and reference mitogen. Taken together, the results demonstrate that HFAE potently activates the immune function by regulating NO, TNF-${alpha}$, IL-$1{eta}$, and IL-6 in RAW 264.7 macrophage and promoting spleen cell proliferation.