OBJECTIVE To evaluate the remineralization of natural human dentin caries with an experimental whisker-reinforced Atraumatic Restorative Treatment (ART) composite. METHODS Teeth with moderate active dentin caries were prepared with caries-disclosing dye and hand instruments, restored with ART or resin-modified glass ionomer cement (RM-GIC), and then wet sliced into 120-µm sections with 15 sections in each group. After taking transverse microradiographs and implementing digital image analysis to determine the "mineral-loss-before," each section was incubated in artificial saliva solution (pH = 7.0) for 4 weeks and 8 weeks with 1 hour each workday in demineralization solution (pH = 4.3). Transverse microradiographs of each section were retaken, and the "mineral-loss-after" was determined. The remineralization was calculated from [1-("mineral-loss-after"/"mineral-loss-before")] × 100%. Results were statistically analyzed with a repeated-measures ANOVA with one within-subject factor (time: 4 and 8 weeks) and one between-subject factor (material: ART and RM-GIC) (α = 0.05). RESULTS The statistical analysis indicated that ART composite resulted in significantly higher remineralization than the RM-GIC (p ≤ 0.05). For the remineralization of each material, there was a statistical difference between 4 weeks and 8 weeks (p ≤ 0.05). CONCLUSIONS This experimental ART composite remineralized natural human dentin caries better than the RM-GIC.