Abstract: Significant coupling between the longitudinal modes with the lower anti-symmetric vertical modes was observed in vibration of suspension bridges. Based on the deflection theory and energy principle, this type of coupling vibration was investigated. First, by assuming the suspended structures as a simple pendulum, the pure longitudinal vibration analyses were carried out to determine the length of equivalent pendulum and the frequency. Then, according to the features of the longitudinal motion associated with the lower vertical modes of the cable, a simplified model was established for stiffening girder longitudinal vibration, in which the effect of the cable motion was taken into account. From Hamilton's Principle, the formula for coupling vibration frequencies was theoretically derived. Finally, investigations on five specific bridge examples indicate that the calculated coupling frequencies and mode shapes were in good agreement with those predicted by finite element method.