So far, many researchers have attempted to unveil the micro-mechanical features of calcium-silicate-hydrate (C-S-H) gel that contributes the most to the engineering performances of cement-based materials. Due to the highly heterogeneous nature of C-S-H gel present in cement-based materials, the influencing factors of the micro-mechanical properties of C-S-H gel are still not clear. This study aims to provide an insight into the factors that govern the micro-mechanical properties of C-S-H gel by investigating the morphology, composition, and micro-mechanical properties of C-S-H gels present in a relatively simple system, ie, a C3S paste. Through this study, it is revealed that the reinforcement by the crystal phase and the size of C3S clinker influence the micro-mechanical properties of inner product C-S-H gel.