Hardening effect is often observed in either experiments or simulations. And several continuum models or semi-empirical theories have been proposed to explain the origin, such as constraint-counting theory, the bond-order-length-strength (BOLS) correlation mechanism, equations of state-Murnaghan relationship, etc. However, the validity of these models or theories at the nanoscale have not been tested. In this work, high-speed head-on impact between silicon nanoparticles were studied using molecular dynamics (MD) simulations and their contact mechanics behaviours including contact force versus nominal displacement relationship were explored and the pronounced hardening effect was clearly observed. That’s, apparent Young’s modulus yielded is much higher than that of their bulk materials. The structure of silicon nanoparticles after compression was analysed in terms of bond length, bond angle, coordination number. The three existing relevant models were separately examined. The results show that any single of the three known theories cannot explain the higher elastic modulus obtained in present MD simulation. Probably, the three aspects contribute together to the hardening effect. This area awaits much more mature theory to explain the hardening effect under the influence of the dynamic effect.