The basic equations are proposed for plastic deformation of porous metals such as sintered metal powders or metal powder compacts. The porous metal is assumed to be made up of a homogeneous metallic material and unformly distributed pores. The following equations are obtained from the Levy-Mises equatioins, (dε1-dε2)/(σ1-σ2)=(dε2-dε3)/(σ2-σ3)=(dε3-dε1)/(σ3-σ1)=(3fdευ)/(2σm/f)=(3dεeq)/(2γσeq) where γ is the relative density of the porous metal, f a function of γ, the subscripts 1, 2 and 3 represent the principal directions, σm=(σ1+σ2+σ3)/3 and dευ=dε1+dε2+dε3. The theory shows a good agreement with the experimental results for sintered copper powder compacts.