Quasi-periodic motion is a kind of common phenomenon in practical engineering. It is also one of the ways from periodic motion to chaos. In this paper the incremental harmonic balance (IHB) method with multiple time scales is presented for the study of quasi-periodic motions of nonlinear systems. Two examples of Duffing and van der Pol-Duffing equations

quasi-periodic motions characteristics of the systems which subjected to the external multi-excitations are obtained. Moreover

the results obtained from the IHB method agree well with those from the numerical integration method using the fourth-order Runge-Kutta method. The numerical examples illustrate the IHB method with multiple time scales can be used to study quasi-period motions of nonlinear systems. It provides an effective method for the further study in nonlinear vibration field.