Regarding the significance of Portland cement paste in construction and building materials, this work aims to study the mechanical properties of Portland cement paste at the micro- and macro-scale, and provide a scientific basis for improving the mechanical performances. To fulfill the aim of this work, a comprehensive experimental and theoretical micromechanical investigation was performed on Portland cement paste. The micromechanical properties of Portland cement paste were investigated by the coupled nanoindentation and scanning electron microscope method. The mechanical parameters (e.g. indentation modulus and hardness) of individual active phases present in Portland cement paste were estimated by the proposed method. From the mechanical parameters of Portland cement paste at the micro-scale, the evolution of elastic modulus of Portland cement paste at the macro-scale was predicted by a homogenization method and the geometries of the main constituents in Portland cement paste were taken into account. After verified by the experimental data, it was found that the proposed method can provide an accurate prediction of early-age elastic modulus of Portland cement paste.