A series of industry-scale and laboratory-scale W-Cu composites containing copper of 19-51 vol% were prepared by infiltrating a tungsten skeleton with liquid copper. These composites were subjected to a three-point bend test at temperatures between 77 and 363 K, and yield and maximum strengths were evaluated as a function of both the temperature and the composition. Fracture surface was observed using SEM and its fracture mode was examined. Results are summarized as follows. (1) Critical Cu content for the transition from a brittle to a ductile deformation behavior at room temperature was about 20 vol%. (2) Critical Cu content for the transition from a bcc-type to a fcc-type deformation behavior was 38-45 vol%.