The aim of the present study is to examine and compare the thermal and mechanical properties of epoxy resin/TiO2 particle microcomposites (0.2m) and nanocomposites (21nm). Composite materials consisting of epoxy resin reinforced with different amounts of TiO2 microparticles (1, 5, 10, 15, and 20%wt) and TiO2 nanoparticles (0.5%, 1%, 3%wt) were prepared. The thermal and mechanical properties of the manufactured composites were investigated and compared through differential scanning calorimetry (DSC) and three-point bending tests (3PB). Lipatov's Theory was then applied on the DSC results, thus leading to the calculation of the particle-matrix interphase thickness which was correlated to experimental findings. The glass transition temperature (T-g) of the materials was obtained and the effect of the grain size on the measured T-g values was investigated. The data obtained from DSC tests for both micro- and nanoinclusions when normalized with respect to the specific surface area of the particles, resulted in a single continuous curve describing the normalized phase transition enthalpy variation with filler weight fraction. POLYM. ENG. SCI., 58:1146-1154, 2018. (c) 2017 Society of Plastics Engineers