The experiments of cold pressing of cottonseed and castor bean were carried out to study the stress-strain relationship and the stress-compression ratio relationship. The effects of rate of applied pressure on the stress-strain relationship and the stress-compression ratio relationship were investigated. The methods of identification of critical pressing pressure were proposed. Hyperbolic stress-strain model and calculative model of compression ratio of oil plant were developed on the basis of the Kawakita's Equation. The results of the experiments indicate that effects of rate of applied pressure on the broken stress and strain and stress-strain relationship are obvious, the broken stress and strain increase with increasing rate of applied pressure. The simulated results of cottonseed and castor bean coincide with those from experiments. By using of variation rate and slope curves of realistic compression ratio, 70 MPa and 100 MPa are defined as the critical pressing pressure of cottonseed and castor bean respectively.