To address the structural complexities of high entropy alloys (HEAs), we have applied atomistic modeling methods to predict the structures of HEA nanoparticles. In our atomistic simulations, the interatomic interactions were described with the modified embedded atom method (MEAM). Using a combined molecular dynamics (MD) and Monte Carlo (MC) simulations based on the developed MEAM potentials, we investigated the formation of solid solution phase in Co0.12Ni0.14Ru0.43Rh0.30, Ru0.44Rh0.30Co0.12Ni0.14, and Ru0.25Rh0.25Co0.2Ni0.2Ir0.1 nanoparticles with size ranging from 2 to 5 nm. Moreover, we used the different duration between MC and MD simulations to model slow annealing and fast quench processes. Our simulation results indicated that the local severe lattice distortion could block the diffusion of atoms and hence lead to a stable solid solution phase during a carbothermal shock synthesis procedure. Consequently, we have demonstrated that atomistic simulation techniques as useful methods for understanding the composition-structure-property relation of novel high entropy alloys.