This study aimed to investigate the use of 38% silver diamine fluoride (SDF) as a treatment for preventing secondary caries in glass ionomer cement (GIC) and composite resin (CR) restorations.Six extracted human sound premolars were collected. Four cavities (4 × 2 × 2 mm(3) ) were prepared on each premolar and then allocated to the following restoration groups: group 1, SDF conditioning and GIC restoration; group 2, GIC restoration; group 3, SDF conditioning and CR restoration; and group 4, CR restoration. After thermal cycling and sterilisation, the teeth were soaked in a 5% sucrose solution containing Streptococcus mutans and Lactobacillus acidophilus for 28 days. Micro-computed tomography was used to study demineralisation. The outer lesion depth (OLD) and wall lesion depth (WLD) of the tooth-restoration interface were measured. The OLD and WLD were directly related to the extent of secondary caries. Two-way analysis of variance was used to analyse the effects of SDF conditioning and restorative materials on OLD.The mean ± standard deviation OLD values were 156 ± 45 μm, 235 ± 33 μm, 153 ± 20 μm and 232 ± 24 μm for groups 1-4, respectively. The OLD was less in restorations with SDF conditioning (P < 0.001) than in those without SDF conditioning. No interaction effect on OLD was found between the restorative materials and SDF conditioning (P = 0.062). The WLD was detected only in groups 3 and 4.Conditioning with 38% SDF can increase resistance of GIC and CR restorations to secondary caries.