Abstract: A total of 260 day-old AA male broilers were randomly assigned to four light treatments to investigate the effect of monochromatic light on immune function. Chicks were exposed to blue (480 nm, B), green (560 nm, G), red (660 nm, R) and white (400-700 nm, W) light by using lightemitting diodes (LED) lamps. All light sources were equalized on the light intensity of 15 lx and applied for 23 h daily. There were five replication pens for each light treatment and 13 birds per pen, and the experiment lasted 7 weeks. On day 49, spleen weights were determined. T lymphocyte proliferation and serum levels of cortisol were analysized on the 21- and 49-day old. Antibody titers to NDV were measured on day 14, 28, 42 and 49. The results were as follows: (1) Blue light significantly increased spleen weight compared to RL groups (42.2%), and no significant difference was found between WL, GL and BL groups at 49 d of age (P<0.05);(2) Blue light significantly decreased cortisol concentrations in serum compared to white light at 21d and 49 d (25.2％and 26.2％ respectively,P＜0.05);(3) A significant elevation in serum anti-NDV level was observed in broilers reared under GL compared to RL at 28 d of age (32.9%), but no significantly difference was detected between BL, RL and WL groups (P>0.05). By 49 d of age, the antibody titers from BL was higher than that of RL (62.8%, P<0.05), but not significant difference among BL, GL and WL groups (P>0.05); (4) A significantly increase of peripheral blood T lymphocytes proliferation was observed in birds reared under WL and GL groups compared to RL at 21 d of age(P<0.05, 101.1％ and 80.8％ respectively). In addition, broilers reared under WL and BL had significantly elevated proliferation response compared to RL at 49 d of age by 41.5％ and 26.9%, respectively (P＜0.05). These results showed that immune function would be increased in broilers when who was illuminated with either green light in early stage of broiler growth stage or blue light in latter growth stage under 15 lx light intensity. Moreover, blue light could, to an extent, show an action of alleviating immunologic stress.