Thermodynamic Study of Grain Growth in Nanocrystalline Structure of Metals with Atomistic Simulations In recent years, the importance of nanocrystals has attracted the attention of many researchers. The reason is the unique properties of nanocrystals compared with conventional materials. The main origin of their excellent properties is the high fraction of grain boundaries and consequently high energy content. On the other hand, these materials have no proper stability even at room temperature and reduce their energy through grain growth. Therefore, thermodynamic study of the grain growth and research about how we can stop this process and stabilize the nanocrystals to keep their unique properties, are very important. In this research, for the study of grain growth process in nanocrystalline metals, the dilated crystal model was used to obtain grain boundary’s thermodynamic parameters. Then, the free energy of grain boundary was calculated versus excess volume for nanocrystalline Palladium, using Equation of State (EOS), Quasi-harmonic Debye Approximation (QDA) and Song’s models. Plotting of thermodynamic diagrams based on EOS and Song’s models showed that by reducing the grain size of Palladiumtoa critical value, the nanocrystal becomes metastable and the grain growth stops. But, based on QDA model, the stoppage of grain growth was impossible. To evaluate these results, Monte Carlo and molecular dynamics simulation methods were used. Results of both methods, confirmed the predictions of EOS and Song’s models and rejected the results of QDA model. Also, the study of temperature effect on the mentioned diagrams showed that by the increase of the temperature, the critical grain size is increased and the stoppage of grain growthis facilitated. Subsequent simulation results confirmed this procedure and showed that EOS model has more accurate predictions compared with Song’s model. Then, the critical temperatures of nanocrystalline Palladium for the stoppage of grain growth in each grain size was calculated using simulation. Also, comparing these results with the EOS results, a modified equation was proposed for the correlation of the critical excess volume and the critical grain size. To ensure the results obtained from the performed simulations, poly-crystal simulations also was carried out via two different methods and the results showed that Voronoi tessellation method for the generation of poly-crystals has no proper outcome suitable for grain growth simulations, but the results of melt growth method matches the previous results. Finally, the grain growth in poly-crystalline iron was simulated. The results showed that the Palladium’s obtained results are true for iron, too.