Abstract: 400 one-day-old AA broilers were divided into four groups randomly . They were added 0,20,40 and 60 mg/L carnosine respectively in the drinking water at 1 to 4 groups. The diameter of fiber of breast muscle and leg muscle were determined on each weekend, and to observe the effects of the carnosine on development of muscle fiber diameter in broilers. The results showed that the breast and leg muscle fiber diameter of the carnosine experimental group were all bigger than the control group at one-week-old, but the difference wasn’t obvious (P >0.05). At two-week -old, breast fiber diameters of the moderate and high dose group were obviously or extremely bigger than the control group respectively(P<0.05,P<0.01).The leg muscle fiber diameters of the high dose group were extremely bigger than the control group (P<0.01). At three-week-old, breast muscle fiber diameters of the high dose group were extremely bigger than low group and the control group (P<0.01). The leg muscle fiber diameters of the high dose group was obviously bigger than the control group(P<0.05). At four-week-old, breast and leg muscle fiber diameters of smaller dose group were both smaller than the control group obviously (P<0.05). At five-week-old, breast fiber diameter of the moderate dose group was smaller than the control group obviously (P<0.05), the leg fiber diameter of the low and moderate dose group were extremely or obviously smaller than the control group respectively(P<0.01,P<0.05). At six -week-old, the muscle fiber diameters of the carnosine experimental group were all smaller than the control group, but the difference wasn’t obvious (P>0.05).However, muscle fiber diameters dealt with carnosine of which the development speed was faster than that of the control group at the 1 to 3 weeks- old while it was lower than that of the control group at the following three weeks in different doses.There were obvious dose effects in all groups with the carnosine at the 1 to 3 weeks.