This paper presents a new voice conversion method called Weighted Frequency Warping (WFW), which combines the well known GMM approach and the frequency warping approach. The harmonic plus stochastic model has been used to analyze, modify and synthesize the speech signal. Special phase manipulation procedures have been designed to allow the system to work in pitch-asynchronous mode. The experiments show that the proposed technique reaches a high degree of similarity between the converted and target speakers, and the naturalness and quality of the resynthesized speech is much higher than those of classical GMM-based systems.