Objective To study the effect of fluoride varnish on microconsistency of enamel after under irradiation in vitro. Methods Normal enamel samples were randomly divided into four groups: control group,irradiation group,fluoride varnish group and fluoride varnish-irradiation group.The microconsistency of enamel surface before and after treatment,after anti-caries test was measured by type MH-5 durometer. Results Fluoride varnish had an inhibitory effect on the decrease of enamel microconsistency etched with lactate buffer.The microconsistency in irradiation group was obviously lower than that in control group(P