A matrix perturbation method of complex modes in vibration analysis of damped system is studied. By introducing a simple normalization condition

the perturbation terms of eigenvectors can be determined conveniently and uniquely. And the perturbation formulas of the first and second order are derived for the cases of distinct and repeated complex eigenvalues. Illustrative examples are given to show that this perturbation method is able to give approximate results extremely close to exact results.