This study was intended to determine the modulatory effects of dietary supplementation of purple sweet potato (Ipomoea batats Poir., PSP) on the immune response of chickens. PSP was included in a basal starter diet by 1% (PSP(L)) or 3% (PSP(H)) and continually fed. Newcastle disease (NDV) vaccine, Brucella abortus (BA) and sheep red blood cells (SRBC) were used for chicken immunization. Antibody titers against these antigens were used to estimate humoral immunity. Concanavalin A (Con A)-induced proliferations of splenocytes, thymocytes and peripheral blood lymphocytes (PBL), ratios of CD4- and CD8-single positive and CD4-CD8-double negative (DN) cells in splenocytes, were both used to indicate cellular immunity. Relative weights of spleen, thymus and bursa and white blood cell (WBC) counts were studied. PSP(H) increased anti-NDV (P < 0.05), anti-BA (P < 0.01) and anti-SRBC titers (P < 0.05) in response to secondary immunization, whereas PSP(L) increased titers of anti-BA (P < 0.05) and anti-SRBC (P < 0.01). Proliferations of splenocytes and thymocytes were augmented with PSP(L) (P < 0.05). PSP(H)-treated chickens had lower (P < 0.05) ratios of CD4-single positive lymphocytes. Proliferation of PBL, weights of lymphoid organs and WBC counts were not affected. These results suggest that dietary PSP supplementation could enhance the immune response after immunization in chickens.