Introduction: Resin modified glass-ionomer cement (RMGI) is prepared by  manual mixing of powder and liquid. Different mixing ratios influence on the RMGI properties. The aim was to compare the effect of different mixing ratios on the microleakage of RMGI.

Materials &Methods: In this in vitro study, 60 Class V cavities (3×2×1.5 mm) with the gingival margin of 1 mm apical to the cement-enamel junction were prepared on the buccal and lingual surfaces of 30 sound premolars. The teeth were randomly divided into 6 groups. Group 1: The manufacturer’s recommended ratio, without conditioning; Group2: The manufacturer’s recommended ratio with conditioning; Group 3: 20% lower than the manufacturer’s ratio without conditioning; Group4: 20% lower than the manufacturer’s ratio with conditioning; Group 5: 20% higher than the manufacturer’s ratio without conditioning; Group6: 20% higher than the manufacturer’s ratio with conditioning. After thermocycling, the microleakage was evaluated using silver nitrate staining. The teeth were cut into two mesial and distal halves, and the microleakage at occlusal and gingival margins was recorded based on a 0‒3 scoring system under a stereomicroscope. Data were analyzed using Kruskal-Wallis and Mann-Whitney tests with significance level at P<0.05.

Results: The maximum microleakage at gingival margins was recorded for group 4, which was significantly higher than that of group 2 and 6 (P=0.043 and P=0.043, respectively). No significant differences were observed in the microleakage between occlusal and gingival margins.

Conclusion: A 20% reduction in P/L ratio of RMGI increases the gingival microleakage when surface conditioning was appli.