Abstract: The effect of monochromatic light on muscle growth and muscle fiber development in broilers were studied, and the internal mechanism was discussed in this paper. A total of 260 newly hatched broilers were divided randomly into four light treatments, and were reared under blue (480 nm, BL), green (560 nm, GL), red (660 nm, RL) and white (400—760 nm, WL) light by using LED for 49 days, respectively. The developmental characters of muscle fiber and the concentration of testosterone were investigated by histological chemistry and radioimmunity. The results showed that: ① At 21 days of age, weight of breast muscle and thigh muscle were significantly higher in GL group than that in other groups (6.46%—13.57% more in breast muscle and 6.37%—16.34% more in thigh muscle, P<0.05). Until 49 days of age, weight of breast muscle and thigh muscle were significantly higher in BL and GL group than that in other groups (21.42% more in breast muscle and 16.67%—25.44% more in thigh muscle, P<0.05). ② At 21 days of age, the area of major pectoral muscle fiber in GL group and gastrocnemius muscle fiber in GL and RL group were higher than that in other groups (Increased by 23.19%—54.01% and 33.46%—56.67% resepectively, P<0.05). Until 49 days of age, the area of major pectoral muscle and the gastrocnemius muscle fiber were higher in BL group than that in other groups (Increased by 9.92%—21.69% and 21.11%—57.44% resepectively, P<0.05). ③ At 21 days of age, the density of major pectoral muscle fiber in RL group and gastrocnemius muscle fiber in BL group were greater than that in others (Increased by 19.17%—42.94% and 10.13%—49.33% resepectively, P<0.05). Until 49 days of age, the density of major pectoral muscle fiber in WL and RL group and gastrocnemius muscle fiber in RL group were greater than that in others (Increased by 27.36% and 57.02%—112.03% resepectively,P<0.05). ④ At 21 days of age, the testosterone concentration was 35.37%—37.13% greater in BL and GL group than that in other groups (P<0.05). By 49 days of age, the testosterone concentration was 14.36%—28.77% greater in BL group than that in other groups (P<0.05). These date indicated that green or blue monochromatic light can be used effectively to increase broiler’s muscle growth and to elevate testosterone secretion.