The viable cell count of monobacterial plaque of Streptococcus and mixed bacterial plaque of streptococcus and Neisseria Veillonella,also the demineralization caused by the plaques were observed.The assumption that Neisseria and Veillonella might reduce demineralization of enamel caused by streptococcus was confirmed.The relationship between streptococcus and Neisseria,Veillonella were preliminarily found out.