Abstract: The aim of this study was to investigate the effects of Enterococcus faecium B21 on body weight, fasting blood glucose levels, and fecal bacterial communities of high fat diet (HFD) -treated Wistar rats. Thirty-six Wistar rats were randomly divided into three groups, including the normal chow-diet (NCD) group, HFD group, HFD and E. faecium B21 (HFD+B21) group. Each rat in the HFD+B21 group was fed with 0.2 mL E. faecium B21 (10⁸CFU/mL) each day. After the intragastric administration for 7 weeks, the feces of the rats were collected, and their fecal bacterial communities were detected by high-throughput sequencing of bacterial 16S rRNA gene. The results showed that body weight of the rats in the HFD+B21 group (366.51 g) were increased significantly compared with those in the groups HFD (339.43 g, p<0.05) and NCD (306.84 g, p<0.05), and the fasting blood glucose levels of rats in the HFD+B21 group (7.63 mmol/L) were reduced significantly compared with those in the HFD group (8.75 mmol/L, p<0.05) and there was no significant difference compared with those in the NCD group (7.08 mmol/L, p>0.05). The HFD+B21 group had the lowest bacterial abundance (ACE-index 398.12, Chao1-index 398.49) and diversity (Shannon index 3.83), while the NCD group had the highest abundance (ACE-index 700.57, Chao1-index 698.07) and diversity (Shannon index 5.06). The results also displayed that the intake of E. faecium B21 increased the ratios of probiotics Blautiain the bacterial population, and inhibited the growth of pathogenic bacteria Pseudomonas and Desulfovibrio. These results suggested that E. faecium B21 had a hypoglycemic effect and regulated the imbalance of intestinal bacterial population in Wistar rats caused by the intake of HFD.