Although nano polycrystalline diamond (NPD) is found with hardness equal to, or even higher than that of single crystal diamond, there are still some controversies about its mechanical responses as well as the plastic deformation mechanisms during the indenting processes. Through molecular dynamics simulations, we have investigated the effects of grain size and temperature on the indentations of NPD. Our results indicate that when the grain size is smaller than 10 nm, NPD would soften as the grain size decreases. For the studied grain sizes of 5, 7, and 10 nm, plastic deformations are resulted from the dislocation propagation mode and the atomic disordering mode. For NPD with the studied grain size of 2 nm, its plastic deformation is mainly determined by the atomic disordering of the diamond grains. Our simulations reveal that higher temperatures result in the softening of NPD, and the plastic deformation mechanisms at high temperatures are basically the same with that at 300 K. The results presented in this work not only provide an answer for the questions about the grain size and temperature effects on the mechanical responses of NPD, but also offer insights into plastic deformation mechanisms of NPD, which could provide reference data for the potential applications as well as the design of novel nano structured superhard materials.