To evaluate the clinical cariostatic efficacy of a concocted 5% Nano-silver incorporated Sodium fluoride (NSSF) dental varnish with 38% Silver diamine fluoride (SDF) in preventing the progression of dentinal caries of primary molars. A total of 159 active dentinal carious lesions in primary molars (from 50 children) were selected and randomly divided into two groups; NSSF and SDF. Varnish application was performed at the baseline visit and follow-up is carried out at 1, 3, 6 and 12 months. Parameters such as caries activity, depth, size, colour, and presence or absence of pain were noted at baseline and follow-up visits. The intergroup comparison of caries activity status did not reveal significant difference between the number of active and arrested caries lesions in NSSF and SDF groups at any visit (p >0.05) during the 12-month follow-up. There was no difference between overall failure and success rates between the two groups at any follow-up period (p-0.48). Annual application of 5% NSSF is better than or equal to 38% SDF in preventing the progression of dentinal caries of primary molars. NSSF does not cause dark staining of dentinal tissue compared to the SDF and the use of NSSF can be recommended in children.