W-Cu composite powders can be prepared by mechanochemical process, where the $WO_3$-CuO composite powders were mechanically synthesized from the elemental oxide powders and subsequently reduced to W-Cu composite powders. In the present work, reduction behavior of$ WO_3$-CuO composite powders that were synthesized at different milling time was examined in terms of hygrometric analysis. In case of $WO_3$-CuO ball-milled for 20 h, the reaction temperature of CuOlongrightarrowCu became lower than in case of 1 h. Also, the reaction of $WO_3$longrightarrow$WO_{2.9-2.72}$ and $WO_{2.9-2.72}$ longrightarrow$WO_2$were shifted to lower temperatures and the peaks were changed to much sharper shape. While the reaction of $WO_2$longrightarrowW in case of ball-milling for 20 h started at lower temperature, the peak temperature was the same as in 1 h ball-milling. The reduced W particle size was somewhat finer fer 20 h ball-milling. It was considered that the refinement of oxide particles caused by ball-milling process leads to such a change in the reduction behavior.