Objective: The hypothesis to be tested was that the clinical performance of nano-filled resin-modified glass ionomers (RMGIs) in Class I primary molars will differ from that of its preceding conventional RMGI. Method and Materials: The shear bond strengths of each material to primary molar dentin were measured (n = 10) using a notched-edge crosshead. Independent sample t test was used for statistical analysis. A split-mouth designed clinical trial was performed in which nano-filled RMGI (Ketac Nano; KN) and conventional RMGI (Vitremer; VR) were used to restore Class I cavities of primary molars. Each material group (n = 30) was evaluated according to United States Public Health Service (USPHS), regarding marginal discoloration, marginal adaptation, color match, anatomic form, and recurrent caries, at baseline, after 1 year, and after 2 years. Wilcoxon signed-rank test and chi-square test were used for statistical analysis. Results: Shear bond strength (mean ± SD) of KN (6.3 ± 3.9 MPa) was significantly lower (P .05) than that of VR (9.5 ± 2.7 MPa). After 2 years, KN restorations showed significantly increased wear (92% Alpha and 8% Charlie) and marginal discoloration (23% Bravo and 8% Charlie) with secondary caries (8%). Color match was significantly decreased for both KN and VR (69% and 73% Bravo, respectively). When parameters were compared for materials at each recall, there was no statistically significant difference between KN and VR. Conclusion: Nano-filled RMGI may not be better than conventional RMGI; its wear resistance deteriorates with time, and it has low bond strength to dentin. Keywords: bond strength, Class I, clinical trial, nano-filled resin-modified glass ionomer, primary teeth