The Cu - $Nb_3Sn$ composites wire as a superconducting material was prepared by in situ method as follow: Cu - 15wt.% Nb alloys which were melted in a high -frequency induction furnace and casted in bar were cold-worked up to the final diameter of 0.24 mm, electroplated with Sn, pre-treated in two steps and then diffused at $550{sim}650^{circ}C$ for 24 ~ 96 hrs. The overall $J_c$ and $T_c$ of the specimens were measured by the four point-probe method at 10 K in the magnetic field of 0 Tesla. The overall $J_c$ of the composites wire which diffused at $550^{circ}C$ after pre-treating in two steps were generally higher than those of the wire at either $600^{circ}C$ or $650^{circ}C$. For the specimens diffused at $550^{circ}C$, the overall $J_c$ were increased until 72 hrs. of diffusion time and then decreased. However, in case of diffusion at $600^{circ}C$ and $650^{circ}C$, the overall $J_c$ were gradually decreased from the beginning. The maximum overall $J_c$ obtained in this experiment was $1.3{ imes}10^4;A/cm^2$, which was measured for the specimen diffused at $550^{circ}C$ for 72 hrs. When the specimens were diffused at $550^{circ}C$ for 72 hrs, after pre-treating, the measured critical temperature, $T_c$ was 16.19 K. Similar $T_c$ value were obtained in other specimens regardless of diffusion time and temperature.