In this paper, the perturbation technique is introduced into the method of harmonic balance. A new method used for analysing nonlinear free vibration of multidegree-offreedom systems and structures is obtained. The form of solution is expanded into a series of Small parameters and harmonics, so no term will be lost in the solution and the algebric equations are linear. With the linear transformations，the matrices of the equations become diagonal. As soon as the modes related to linear vibration are found，the solution can be obtained. This method is superior to the method of linearized iteration. The examples show that the method has high accuracy for small amplitude problems and the results for rather large amplitude are satisfactory.