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Vehicle dynamic control could improve vehicle performance. Vehicle stability is vital to the determination of vehicle dynamic control strategy. The phase plane method is one of the most common methods to judge vehicle stability. To determine the 4WS (four-wheel steering) vehicle stability status faster and more accurately, a novel method to assess the vehicle stability is based on the vehicle sideslip angle and angular velocity (β-β˙) phase plane. At first, the 2 DOF (degree of freedom) model with a nonlinear tire model is established to acquire β-β˙ phase plane. Then the boundary of the stability region generated by the current method is compared. A crosspoint-ellipse method is provided based on the boundary comparison with the ideal boundary. The boundary function determined by the crosspoint-ellipse method is fitted based on vehicle dynamic theory and the boundary analysis with different steering angles, velocity, and road adhesion coefficient.