Abstract: A numerical approach is developed for natural vibration analysis of turbomachinery disk with small mistuning blades by using modal synthesis technique.Due to lack of rotational periodicity,the group theory method for the tuned bladed disk assemblies is not applicable to the mistuned ones.According to the matrix peturbation theory,the modes used for synthesis in this paper are taken from a tuned bladed disk corresponding to the small mistuning one.This new technique keeps the advantages of the group theory method and makes the final eigenvalue problem of small size.A computer program has been written to analyse the natural vibration characteristics of turbomachinery disk with small mistuning blades.The computational results prove that the method is very efficient.