Nanoscale polycrystalline materials with unique mechanical properties become now a hot issue in the field of materials science. One of the research focuses is the grain size effect on the material mechanical properties. In this paper, the grain size effect of TiN nanocrystals is studied with the first principle method based on the density functional theory. The total energies of a series of TiN crystals with different grain sizes ranging from 0.6387nm to 2.332nm are calculated by means of the first principle method to reveal their mechanical properties in the case of simple tension. The calculation results include the strain-stress relationships and the related yield strength. The strain-stress curves show that the yield strength of TiN crystal decreases with the increase of the grain size. That means that the TiN crystal trends to softening as the result of the increase of the grain size. The stress-strain curve indicates also that TiN crystal begins to yield at the tensile strain of 5%, and the yield strength is about 21.5GPa. The TiN crystal has the highest tensile strength at approximately 15% of the tensile strain. As the grain size increases, the tensile strength of TiN decreases. The correlation between the microstructure of TiN and their hardness and elastic properties is discussed, which indicates that besides the grain size effect, the defects in the TiN film is a crucial factor that affects their hardness and strength.