Abstract: The authors analysed the gum flora of rats qualitatively and quantitatively by magnetic bar dilution method. The results showed that in conventional and SPF rats the dominant bacteria were: micrococcus, streptococcus，corynebacterium and propionibacterium，with isolation rate more than 70%, The results suggest that the SPF rats are more suitable for oral disease study as they are less in individual difference in gum flora than those of the conventional ones.