The effects of stacking fault (SF) and temperature on the mechanical properties of nano-polycrystal Mg under tension loading are investigated by molecular dynamics simulations. The interatomic potential of embedded atom method (EAM) is used as the Mg-Mg interaction. The computational results show that the yield strength of nano-polycrystal Mg can be obviously enhanced when stacking fault is introduced into grains, and the effect of SF on the Young's modulus of nano-polycrystal Mg is very small. The results also show that tensile twins and new grain at 300.0 K are nucleated and initiated at grain boundaries, growing continuously with the increase of strain. The dihedral angel between the (1000) plane of new grain and the X-Y plane is about 35. In other words, the nucleation and the growth of twins and new grains are the predominant deformation mechanism for nano-polycrystal Mg at 300.0K. We also find that at 10.0K the dislocation nucleation and slip are the predominant modes of the plastic deformation for nano-polycrystal Mg.