Aim: The aim of this study was to evaluate the microtensile bond strength of a low viscosity flowable resin composite and two different highly filled resin composites as core materials. Materials and methods: Three experimental groups with three different resin composites were established (Group I - Filtek Flow/3M ESPE; Group II - Core Max II/Dentsply; and Group III - Filtek Z350/3M ESPE). Twenty beam-shaped 1-mm thick specimens per group were obtained by sectioning from cylinders (10 × 10 mm) resin composite materials built up around the fibre-reinforced composite (FRC) posts conditioned with 24% hydrogen peroxide and silane. Each specimen was loaded with tension on a universal testing machine until failure occurred at either one of the two post core interfaces present in each specimen. The differences in the interfacial bond strength among the groups were tested for statistical significance with one-way ANOVA test, followed by the Dunnet test for post hoc comparisons. Results and conclusions: The statistical analysis revealed that Group I achieved significantly higher bond strength than Groups II and III (P 0.000001). Flowable composites appear to be preferable to light-activated nano-composite or self-cure composite materials for core build-up on a fibrereinforced resin post treated with hydrogen peroxide and silane as regards to bond strength. Keywords: core material, fibre-reinforced composite post, microtensile bond strength, resin composite, surface treatment