This study investigated the effects of spherical silica fillers on the physical and mechanical properties of resin-modified glass-ionomer cement (RMGIC). Specimens were fabricated by mixing untreated (UF) or silanized (SF) spherical silica filler into the powder of a commercially prepared RMGIC. The original RMGIC and a preparation containing 20 wt% spherical silica filler were also examined with regard to their fractured surface and fluoride release. The fillers increased the compressive strength remarkably: up to 17% in the case of SF and 9% in the case of UF. Both UF and SF increased the flexural strength by up to 17%. The addition of SF increased the DTS up to 38%, but UF decreased the DTS. The addition of SF improved the workability and the mechanical properties of the RMGIC.