Molecular dynamics simulation and analysis are performed on the mechanical properties of mono-and poly-crystalline nano-Cu film. Computational results indicate that the temperature effects are very significant on the tensile strength and the deformation mechanism of the mono-crystalline Cu-film. The strength of the sample is much higher at absolute zero temperature than at normal temperature. The plastic deformation at zero temperature is induced by the short-distantce sliding of the particles while it is caused by the long-distance propagation of dislocations at normal temperature. The strength of polycrystalline Cu-film increases with the decrease of grain size. In the process of strain accretion, grain boundary sliding dominates the plastic deformation. Dislocations originating from grain boundaries propagate toward grains. The area of the grain boundary enlarges and the grain size shrinks with the development of plastic deformation. However, the shapes of the basic grains have no remarkable change even at very large strain.