This study evaluated the effects of $gamma$-irradiation on immunomodulating properties and structural changes of ${eta}$-glucan. ${eta}$-Glucan solutions (10 mg/mL) were ${gamma}$-irradiated at 10, 30, and 50 kGy. Splenocyte proliferation and cytokine (interferon-${gamma}$ and interlukin-2) productions by ${gamma}$-irradiated ${eta}$-glucan were evaluated in in vivo and in vitro, and structural changes of ${eta}$-glucan were also determined after ${gamma}$-irradiation. ${gamma}$-Irradiation on ${eta}$-glucan at 50 kGy enhanced splenocyte proliferation and cytokine productions, (p<0.05) and cleft glycosidic bonds of ${eta}$-glucan resulting in lower the molecular weight. These results indicate that the use of ${gamma}$-irradiation on ${eta}$-glucan may be useful for improving its immunological activity by lowering the molecular weight of ${eta}$-glucan.