Tri-axial compression tests were made on copper, zinc and aluminium powders of specified in particle shapes and sizes under hydrostatic confining pressure up to 360 kg/cm2. By the examination of mechanical behaviours of each powder during compaction, the following results were obtained: The principal stress ratio, σ2/σ1, for each powder showed a nearly constant value during compacting up to the maximum pressure of this test. The ratio depended mainly on the powder material and it increased about 5 and 24% for copper and zinc powders respectively, when 1 wt% of zinc stearate was added as a lubricant. The friction angles calculated from these values were about 43°, 44° and 45°, for aluminium, copper and zinc powders in the unlubricated condition. The change in shearing strength of all powders during compaction could be expressed in the form of Mohr-Coulomb yield criterion.