Oral cavity offers a good environment for the bacterial growth. 까le species diversity of oral bacteria has been studied for many years based on the classification of liable bacteria. In order to acquire more comprehensive insight into the baα.erial community of human oral cavity, we used molecular ecological methods to clone 16S rDNAs from human saliva. DNA was direαly extracted from saliva collection of human according to age. By 27F, 1492R primer for 16s rDNA’s amplification were enforced. Cloning was achieved from the amplified DNA using pGEM-T Easy Veαor and competent cellJM 109. 159 different base sequences were obtained from saliva samples of four different persons, chosen according to the age group. In all samples, Streptococcus sp. were the most abundant rnicrobes, followed by Prevotella sp., except in the saliva of an old person where Rothia sp. presented the second dominant group 까lis saliva sample showed another and more irnportant characteristics that there were increased species diversity, from pathogenic bacteria like Haemophilus sp. and lautropia sp. to the higher pr'φ。rtion of unculturable bacteria. The same tendency, but less clearly, was found for the saliva of adult smoker in the forties. These results suggest that environmental factors in the oral caviψ of smoker and older person makes favourable condition for the infectious bacteria.