This study aims to evaluate the remineralization effects of nano-hydroxyapatite paste on the early demineralized enamel under a simulated pH cycling model compared with fluoride paste. Enamel specimens were obtained from human premolars and embedded in acrylic resin blocks. All specimens were ground flat and subjected to demineralized for 7 days to create the early demineralized lesion prior to the perpendicular sectioned through the surface for measuring mineral loss. Forty-eight specimens with an average of mineral loss were selected and randomly allocated to three groups (n=16): control (without treatment), nano-hydroxyapatite paste (10% HAP), and fluoride paste (1000 ppm NaF). Mineral loss (ΔZ) and lesion depth (LD) were analyzed using micro-computed tomography. All specimens were evaluated after assigned treatment and simulated pH cycling model for 7 and 14 days. Within the experimental group, mineral loss and lesion depth were examined at the period of 7 and 14 days with one-way ANOVA. All collected data were calculated for the resulting changes in mineral loss (ΔΔZ) and lesion depth (ΔLD) and analyzed between the experimental groups and times with two ways ANOVA. All statistical analyses were performed at a 5% level of significance. The results showed that the nano-hydroxyapatite paste and fluoride paste exhibited the ability to diminish mineral loss and lesion depth significantly after treated for 14 days. On the other hand, the control group showed progress in mineral loss and lesion depth after a period of 7 and 14 days. In conclusion, the use of nano-hydroxyapatite paste under the condition of pH-cycling for 14 days promotes remineralization on early demineralized enamel in the same way as using fluoride paste. However, the different characteristics of remineralization should be observed.