Abstract: The cause of empty freight car derailment on tangent is analyzed. Authors con sider that the unfitness in the proportion of the vehicle hunting wavelength to that of the cross level irregularity, or the resonance resulted from the equalit y between the vehicle hunting frequency and the frequency of wheel sets or bog ies horizontal natural frequency causes the derailment. Preventive measures ar e proposed. They are to raise the standard of cross level irregularity, especiall y to prevent the continuous and periodical counter wavelength, to improve the st ructure of 8A bogie, to diminish the interval between two rails, and to obtain t he data of the horizontal natural frequency of wheel set and bogie from the manu facturer so as to avoid the resonance velocity and so on.