Calcium-silicate-hydrate (C-S-H) gel, the main product of cement hydration, contributes the most to engineering properties of concrete. Hence, the microstructural physical and mechanical properties of C-S-H gel present in cementitious composites were investigated by the coupled nanoindentation and scanning electron microscope analysis. The physical and mechanical properties were linked through the micro-poromechanical approach. Through this study, an insight was provided into the microstructural features of C-S-H gel present in cementitious composites. It is found that C-S-H gel is a multi-scale composite composed of C-S-H solid, pore and intermixtures at the scale of nanoindentation on C-S-H gel, and the physical and mechanical properties of C-S-H gel can be influenced by the porosity and volume fraction of the intermixtures. © 2014 Elsevier Inc. All rights reserved.