Objective:The purpose of this study was to quantitatively evaluate the demineralization of the caries-vulnerable sites of first permanentmolars, and identify the most caries-susceptible site in caries-active children.Methods:88 caries-active children (dmft\5) were selected in this study, inwhich 42 children are girls and 46 children are boys. Demineralization of central fossa, lingual groove of the bilateral maxillary first permanentmolars, and the central fossa and buccal groove of the bilateral mandibular first permanent molars were evaluated using laser fluorescence system KaVo DIAGNOdent (Germany). Data were processed using software STATA 510.Results:All the tested siteswere at the initial stage of enamel caries. There was no significant difference of demineralization between the same sites bilaterally (P>0105), or between different genders (P>0105). The central fossa of mandibular first permanentmolar demonstrated significantly higher demineralization than other sites of first permanentmolars (P
