This trial was designed to study the effects of crude protein levels in starters on rumen fermentation, enzymes activity of rumen microorganisms and rumen development of weaned calves. Fifteen Holstein bull calves were fed starters containing 16.22％ (low protein content, group LP), 20.21％ (medium protein content, group MP) and 24.30％ (high protein content, group HP) crude protein, respectively. Starters were limited fed to calves from 8 to 16 wk of age. The rumen fluid was taken out by rumen duct at age of 12 wk after feeding 3 h and by euthanized at age of 16 wk before feeding to investigate rumen fermentation, absorbability and enzymes activity. All calves were euthanized at 16 wk of age and rumen tissue was sampled for rumen epithelial development measurements. The results indicated that calves in group HP had low rumen pH at 12 wk but high rumen pH at 16 wk. There were higher ammonia nitrogen concentrations and rumen volatile fatty acid concentrations in calves fed starter with HP than those in calves fed other starters (P>0.05). The α-amylase activity was lower in calves that fed MP starter than that fed other starters (P>0.05). The protease and cellulases activities tended to be increased as the crude protein content increase in starters and was higher in 16 wk than in 12 wk in the same group(P>0.05). The result of rumen development indicated that the rumen relative weight of calves in group MP was the lowest in all calves at 16 wk. The papillae length and rumen mucosa thickness of group LP were higher than that in groups MP and HP (P>0.05). However, the papillae width of the calves fed starter with MP was the greatest. To sum up, There were not significant effects on rumen fermentation, rumen microorganisms enzyme activity and rumen development to calves fed different crude protein levels of starters from 8 to 16 wk(P＞0.05). But high crude protein level seemed to increase the rumen fermentation ability and microorganisms enzyme activity.［Chinese Journal of Animal Nutrition, 2010，22（1）:57-62］