This study aimed at examining if B.bifidum,a major specie of human colonic microflora,participates in the mice ecosystem in the intestinal tract and antioxidant activity by examining the population of different microorganisms and evaluating the reactive oxygen species isolated from feces and intestinal contents over a 28 day time oral administration period.Microbial diversities were assessed by culturing,PCR-DGGE and real-time PCR.This research showed that the intake of B.bifidum significantly(P<0.05)improved the ecosystem of the intestinal tract in BALB/c mice by increasing the amount of probiotics(Lactobacillus intestinalis,Lactobacillus crispatus)and reducing the populations of unwanted bacteria(Enterobacter,Escherichia coli).Antioxidative activities of incubated cell-free extracts were evaluated by various assays,including scavenging ability of DPPH radical(64.5%and 67.54%,respectively,at 21d in Nutrient and 28 d in MRS broth),superoxide anion,hydroxyl radical(84%and 61.5%,respectively,in both broth),determination of total reducing power(234um/L and 231.5um/L,respectively,at 28 d in Nutrient and 14 d in MRS broth)and mRNA level of gene related to oxidative stress.This study suggests that B.bifidum have a beneficial improvement on murine gut microbiotas and their antioxidant activities,compared to controls,and could be considered as a potential bio-resource antioxidant for promoting health.