The Cu-Nb₃Sn 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㎜, electroplated with Sn, pre-treated in two steps and then diffused at 550∼650℃ for 24∼96hrs. The overall J_c and T_c of the specimens were measured by the four point-probe method at 10K in the magnetic field of 0 Tesla. The overall J_c of the composites wire which diffused at 550℃ after pre-treating in two steps were generally higher than those of the wire at either 600℃ or 650℃. For the specimens diffused at 550℃, the overall J_c were increased until 72hrs. of diffusion time and then decreased. However, in case of diffusion at 600℃ and 650℃, the overall J_c were gradually decreased from the beginning. The maximum overall J_c obtained in this experiment was 1.3×10⁴A/㎠, which was measured for the specimen diffused at 550 for 72hrs. When the specimens were diffused at 550℃, for 72hrs. after pre-treating, the measured critical temperature, T_c was 16.19K. Similar T_c value were obtained in other specimens regardless of diffusion time and temperature.