Objective To observe the effects of casein phosphopeptide-amorphous calcium phosphate (CPP-ACP), casein phosphopeptide-amorphous calcium phosphate fluoride (CPP-ACPF), tri-calcium phosphate fluoride (TCP+F), and Duraphat fluoride varnish on surface hardness and surface morphology of eroded primary teeth enamel. Methods A total of 50 samples of retained mandibular primary central incisors (the hardness value was 280-330 HV) were collected and randomly divided into the control group, CPP-ACP group, CPP-ACPF group, TCP+F group, and Duraphat group. All samples were eroded by Coca-Cola for 5 min per time, 5 times a day for 5 d; remineralized by the deionized water (the control group), CPP-ACP, CPP-ACPF, TCP+F, and Duraphat; and then eroded for another 5 d. The surface hardness (microhardness value) of samples before and after treatment was measured by the microhardness tester. The morphological changes of enamel surface were observed by the scanning electron microscope. Results The differences of microhardness values of five groups before erosion treatment were not statistically significant (P>0.05). The microhardness values of each group after erosion treatment were significantly lower than those before erosion treatment (P<0.05), but the differences among groups were not statistically significant (P>0.05). After being eroded for another 5 d, the microhardness values of CPP-ACP group, CPP-ACPF group, TCP+F group, and Duraphat group were significantly higher than those of the control group (P<0.05), but the differences of four groups were not statistically significant (P>0.05). The mineral deposit was observed by the scanning electron microscope in samples of four groups after being eroded for

another 5 days, except samples of the control group. Conclusion Remineralization agents CPP-ACP, CPP-ACPF, TCP+F, and Duraphat fluoride varnish can effectively inhibit the continuous demineralization of eroded primary teeth enamel.