Densification behavior of ceramic powder by pressure at room temperature was investigated using the constitutive equation based on plastic deformation theory for porous materials (the Lee-Kim model). The modified Lee-Kim constitutive model with two parameters is compared with the Shima model (four parameters) and the Cam-Clay model (three parameters and one polynomial equation). The calculated yield loci using the modified Lee-Kim model were compared with the experimental and calculated data for silicon nitride (Si3N4) powder. Contrary to the conclusion of some previous researchers, all three models simulate densification behavior of ceramic powder reasonably well.