To study the effect of grain size on the mechanical properties and deformation behavior of γ-TiAl Alloy in nanoindentation process, a polycrystalline γ-TiAl model was established by Voronoi method, and the nanoindentation process for different grain sizes was simulated by molecular dynamics method. According to the simulation results, the Load-depth curves of different grain sizes were obtained, and the hardness of γ-TiAl alloy with 7 kinds of grain sizes was calculated. The results show that the relationship between grain size and hardness exhibits an inverse Hall-Petch when the grain size is less than 9.9 nm. Meanwhile, the grain boundary activity and dislocation sliding promote the plastic deformation of matrix, and the grain boundary activity plays a major role. However, the relationship between grain size and hardness conforms to Hall-Petch when the grain size exceeds 9.9 nm. The grain boundary has little effect on the plastic deformation, and the plastic deformation of matrix is dominated by dislocation. In addition, the stress transfer and deformation recovery of γ-TiAl were analyzed in the nanoindentation process, it was found that the dense grain boundary grid can effectively inhibit the indentation defects and the internal stress transfer to the matrix. When the grain size becomes smaller, the stress distribution would be more uniform under the indenter and the elastic recovery ratio would be smaller along the indentation direction.