Objective: To evaluate the remineralization effects of the casein phosphopeptide-amorphous calciumphosphate (CPP-ACP) paste on the white spot lesions (WSLs) of the primary teeth thereby to assess its caries-prevention efficacy on early childhood caries (ECC) in vivo. Methods: A total of 11 high caries risk children with 36 noncavitated caries on smooth surfaces in primary incisors and canines were assigned to receive CPP-ACP (GC Tooth Mousse, GC JAPAN) in addition to daily use of fluoridated toothpaste for 4 weeks. Thirty six WSLs on primary incisors and canines were evaluated. During 4 weeks treatment period, all subjects were instructed to use daily fluoridated toothpaste (500 ppm F- as NaF) and additionally applied a CPP-ACP containing paste on respective surfaces for 1 minute, twice a day. Baseline and final mineralization status were determined using a laser-induced infrared fluorescence (FL) device (DIAGNOdent™,KaVoDentalGmbH, Germany ). Results: The mean LF results for WSLs at the buccal surfaces of the primary teeth before CPP-ACP paste application was 8.41 ± 12.43 and after was 1.95 ± 4.69 (P<0.001). CPPACP paste in addition to daily use of fluoridated toothpaste produced an increase in WSLs remineralization of 77% (P<0.001). Conclusion: This 4-week clinical study have indicated that twice daily topical applications of CPP-ACP containing paste as an adjunct to a standard oral hygiene programme which includes fluoridated toothpaste, significantly improve the remineralisation of white spot lesions. The usage of CPP-ACP paste with Fluoride toothpaste could be effective for preventing demineralization and promoting remineralization of enamel subsurface lesions.