Using a physical processing apparatus capable of composite particle preparation under vacuum, the effect of vacuum on the preparation of two sets of mixed powders, Al2O3 guest particles-Cu core particles and Cu guest particles-Al2O3 core particles, was investigated. Variations in the yield of formation of the composite particles, the surface flatness and the compactness of the coating layer were discussed. The temperature rise of the particles during processing was also discussed. Moreover, the surface oxygen content of the coating layer under vacuum or under a low pressure flow of Ar or H-2 gas was considered.