Background: Breast cancer is the most common female malignancy in the world. Beta glucan can be a hematopoietic and an immune modulator agent in cancer patients. The aim of this trial was to determine the effect of beta glucan on white blood cell counts and serum levels of IL-4 and IL-12 in women with breast cancer undergoing chemotherapy. Materials and Methods: This randomized double-blind placebo-controlled clinical trial was conducted on 30 women with breast carcinoma aged 28-65 years. The eligible participants were randomly assigned to intervention (n=15) or placebo (n=15) groups using a block randomization procedure with matching based on age, course of chemotherapy and menopause status. Patients in the intervention group received two 10-mg capsules of soluble 1-3, 1-6, D-beta glucan daily and the control group receiving placebo during 21 days, the interval between two courses of chemotherapy. White blood cells, neuthrophil, lymphocyte and monocyte counts as well as serum levels of IL-4 and IL-12 were measured at baseline and at the end of the study as primary outcomes of the study. Results: In both groups white blood cell counts decreased after 21 days of the intervention, however in the beta glucan group, WBC was less decreased non significantly than the placebo group. At the end of the study, the change in the serum level of IL-4 in the beta glucan group in comparison with the placebo group was statistically significant (p=0.001). The serum level of IL-12 in the beta glucan group statistically increased (p=0.03) and comparison between two groups at the end of the study was significant after adjusting for baseline values and covariates (p=0.007). Conclusions: The findings suggest that beta glucan can be useful as a complementary or adjuvant therapy and immunomodulary agent in breast cancer patients in combination with cancer therapies, but further studies are needed for confirmation.