This experiment was conducted to investigate the effects of different nutritional levels on muscle histological characteristics of lambs. Eighty castrated Mongolian lambs were randomly divided into four equal groups. Lambs in the control group (group C) were fed a diet (ME 10.88 MJ/kg, CP 15.00%) ad libitum; lambs in the moderate restriction groups (groups R1 and R2) were fed restriction diets (ME 10.88 MJ/kg, CP 10.00% and ME 9.41 MJ/kg, CP 10.00%, respectively); lambs in the severe restriction group (group R3) were fed a restriction diet (ME 8.62 MJ/kg, CP 5.70%). The lambs were fed restriction diets for 60 days followed by compensation diet (ME 9.75 MJ/kg, CP 12.00%) for 90 days. The changes in the diameter, cross section area and density of muscle fibers in longissimus dorsi, semitendinosus and biceps femoris of the lambs in each group were measured in the restriction and compensation period. The results showed that the muscle fiber diameter and cross section area of groups R1 and R2 were higher than those of groups C and R3. However, the muscle fiber density of groups R1 and R2 was lower than that of group C, and significantly lower than that of group R3 （P<0.01） with feed restriction. There were no significant differences in the diameter, cross section area and density of muscle fiber among the four groups by the end of the compensation period （P>0.05）. The results indicate that the energy and protein levels of the diets significantly affect the diameter, cross section area and density of muscle fibers in lambs. The development of muscle fiber in lambs is severely affected by the nutritional levels of the diet (ME 8.62 MJ/kg, CP 5.70%) during the restriction period. However, the muscle fiber develops rapidly, and presents compensatory growth effects during the compensation period.［Chinese Journal of Animal Nutrition，2011，23（2）：336-342］