Objective To explore a selected-media of Bifidobacterium from oral cavity, to detect the distribution of Bifidobacterium in different sites of children and primarily investigate the relationship between oral Bifidobacterium and early childhood caries. Methods 70 children aged from 3 to 5 -year -old were selected, 30 children were caries-free and 40 were severe early childhood caries（S-ECC）. Saliva was collected and plaque samples from the 30 healthy subjects were pooled. For S-ECC group, plaques were collected separately from four different sites as follows: Saliva, surfaces of intact enamel, surfaces of white spot-lesions, and deep dentin-lesions. Samples would be grown in the selected-media, and the whole DNA of bacteria was extracted. Polymerase chain reaction was performed with specific primers and the results were analyzed by the electrophoresis. Results Bifidobacterium were detected 0 in the caries-free children, while 47.5％ in the S-ECC group. There was significant difference between two groups（P0.05）. 27.5% Bifidobacterium were detected in saliva, 27.5% on surfaces of intact enamel, 20.0% on surfaces of white spot-lesions and 22.5% in deep dentin-lesions. 10% Bifidobacterium dentium were detected in saliva, 7.5% on surfaces of intact enamel, 7.5% on surfaces of white spot-lesions and 10.0% in deep dentin-lesions. Conclusion One type of modified selected media of Bifidobacterium in oral cavity was explored. Bifidobacterium may be related to the occurrence of the S-ECC and has nothing to do with different sites of teeth in children．
