Different caries prevention trials including fluoride vanish, IgY-passive immunotherapy and probiotics lasting 24 months were conducted with a total of 500 children aged 24 to 54 months of age in Lin'an, Zhejiang, China. The trials were cluster— domized .After obtaining parent´s informed consent, preschool children were clinical examinated according to ICDAS-Ⅱ and microbiological tested of saliva at baseline, 1m, 6m, 12m, 18m and 24m. To find the differences of caries incidence and oral microecology change in four groups, dmfs will be assessed and saliva will be analysed in experiment.