Abstract: To legitimately determine the parameters of pressing cylinder, theoretical formula of lateral pressure coefficient regarding to axial stress of castor seeds was set up through plunger type uniaxial pressing tests on castor seeds, and density of compacted residues of castor seeds, lateral stress and axial stress were obtained. The relation curve about density of compacted residues and axial stress obtained by tests was approximate to the pressing curve of metal powders. Thus we utilized the density formula in the process of pressing metal powders to fit the data, and the relation curve about lateral pressure coefficient and axial stress obtained by tests was built. The maximum relative error of theoretical and measured values was 24.74% and the average error was 7.56%, which proved that theoretical model of lateral pressure coefficient based on the process of deducing the plastic model could well describe the changes of lateral pressure coefficient of castor seeds.