The microstructure of Cu-Mg-P base alloys were significantly affected by small amounts of Fe and Co additions, however the tensile properties and electrical conductivity of the Cu alloys were mainly determined by the fabrication process. Relatively high electrical conductivity (> 80% IACS) was obtained in the all Cu-Mg-P based alloys when they were finally aged at $480^{circ}C$. Tensile properties could be significantly enhanced by final cold rolling, especially at extremely low temperatures. Softening of cold-rolled alloys took place at about $450^{circ}C$ owing to recovery and recrystallization, but it was delayed up to $500^{circ}C$ in the Fe-added alloy.