The equations for the densification control by viscous flow in the hot pressing of metallic glass powders are derived and a criterion for the full densification thereof and an approach for the porosity control are proposed. The kinetic equations are derived on the basis of Arzt's hot pressing theory; in which the densification is modeled by the contraction of the Bolonoi cell of the random dense packing of spherical particles. The constitutive equation for the viscous flow is obtained from the hitherto proposed creep rate equation setting the rate exponent to be unity. The hypothetical constraint for the deformation due to the three grain edges formed in the later stage of densification is taken into account as a modification of the pressure term in the kinetic equations. The relative density is expressed as functions of hot pressing time and the ratio of the viscosity and the pressure. It is shown that the time required for full densification of metallic glass powders depends only on the viscosity/pressure ratio. The criterion for the full densification is expressed as a straight line on the time-viscosity/pressure ratio plane, which divides the area into full-dense and porous areas. The porosity control of the metallic glass powder compacts is also demonstrated as another practical application of the present theory. The results are discussed on the basis of the reported data.