Objective: To evaluate the effect of different particulate nanofiller fractions on the surface microhardness of short glass-fibre reinforced semi-IPN polymer matrix composite resin. Methods: Experimental composite resin (FC) was prepared by mixing 22.5 wt% of short Eglass fibres (3 mm in length) to 22.5 wt% of resin matrix with various weight fractions of nanofillers (0, 10, 20, 30, 40, 50 wt%) and then 55 wt% of silane-treated silica filler was gradually added using a high-speed mixing machine. Three filling composite resins (Z250, Grandio and Nulite), resin-modified glass ionomers (Fuji II LC), amalgam (ANA 2000), fibre-reinforced composite (FRC; everStick and Ribbond), and prefabricated ceramic filling insert (Cerana class 1) were tested in this study. Enamel and dentine were used as controls. The specimens (n = 3 per group) were polished and water-stored at 37°C for 24 h before testing. A universal testing machine was used for testing Vickers microhardness. All results were analysed statistically with one-way analysis of variance (ANOVA). Results: ANOVA revealed that nanofiller fraction had a significant effect (P 0.05) on the Vickers microhardness of the short-fibre composite resin. No statistically significant difference was found between FC composite resin and conventional filling composite resins (Nulite and Z250) (P > 0.05). Ribbond FRC had a lower surface microhardness than everStick FRC (P 0.05). Conclusion: The use of high nanofiller fraction with short-fibre fillers in IPN polymer matrix yielded increased surface microhardness. Palabras clave: surface microhardness, nanofillers, fibre composite resin, filling materials