A low temperature process technology of copper thin films has been developed by a chemical vapor deposition technology for multi-level metallzations in ULSI fabrication. The copper films were deposited on TiN/Si substrates in helium atmosphere with the substrate temperature between $130^{circ}C$ and $250^{circ}C$. In order to get more reliable metallizations, effects on the post-annealing treatment to the electrical properties of the copper films have been investigated. The Cu films were annealed at the $5 imes10^{-6}$ Torr vacuum condition and the electrical resistivity and the nano-structures were measured for the Cu films. The electrical resistivity of Cu films shown to be reduced by the post-annealing. The electrical resistivity of 2.0 $mu Omega cdot extrm{cm}$ was obtained for the sample deposited at the substrate temperature of $180^{circ}C$ after vacuum annealed at $300^{circ}C$. The resistivity variations of the films was not exactly matched with the size of the nano-structures of the copper grains, but more depended on the contamination of the copper films.