This study explores the effects of wheel set gyroscopic action on hunting stability by calculating linear and nonlinear critical speeds. First a dynamic model for a high-speed vehicle with 23 degrees of freedom is developed by considering wheel set gyroscopic action. The linear and nonlinear critical speeds are calculated by eigenvalue analysis and drawing a bifurcation map respectively. Two computer programs for linear and nonlinear stability analysis are developed. Second based on an actual high-speed vehicle in China the effects of wheel set gyroscopic action on hunting stability are quantitatively investigated using computer simulation. Furthermore the difference between the effects of gyroscopic moments about the x-axis and zaxis is discussed. The results show that themomentabout the x-axis is harmful to hunting stability but themomentabout the z-axis is beneficial to hunting stability. However the integrated effect of these two moments can enhance the critical speeds and suppress the hunting motion.