Abstract: The purpose of this study was to assess the effects of dietary supplementation of a compound micro-ecological preparation on growth performance,number of fecal microbes,diarrhea rate and serum indexes of calves.A total of 20 calves aged from 3 to 5 months were selected and divided into experimental group (n=10) and control group (n=10) according to their gender and body weight.The calves in the control group were fed with a basal diet,and those in the experimental group fed a basal diet supplemented with a compound micro-ecological preparation containing Saccharomyces cerevisiae,Bacillus subtilis and lactic acid bacteria.The feeding experiment lasted for 30 days.The calves were weighted at the beginning and end of the experiment,and the total weight gain and average daily weight gain during the experiment of the two groups were calculated and statistically compared.The fecal samples of the calves at the beginning and end of the experiment were collected,and the number of fecal microbes of the two groups before and after the experiment were counted and statistically assessed.The diarrhea incidence of the calves in the two separate groups was recorded and compared.At the end of the experiment,the blood samples of the calves were collected,and the main blood indexes of the two groups were determined and compared.The results showed that the dietary supplementation of compound micro-ecological preparation had no significant impact on growth performance of calves (P>0.05);the calves fed the basal diet added with compound micro-ecological preparation had significantly higher number of fecal lactic acid bacteria and significantly lower number of fecal coliform bacteria compared to the control group (P<0.05),with the number of fecal lactic acid bacteria reaching up to 2.28×10^7 CFU/g;the lower diarrhea rate was observed in the calves of experimental group;there were no significant differences in serum biochemical indexes between experimental group and control group (P>0.05).In conclusion,dietary supplementation of compound micro-ecological preparation can effectively improve the intestinal micro-ecology environment and reduce the diarrhea rate of calves.