This paper presents a novel voice conversion system based on codebook mapping. A new phoneme-tied weighting strategy is proposed to reduce the smoothing effects in weighted sum of code books, while a new prosodic conversion method by decision tree is proposed to cope with the complex prosody of Chinese. STRAIGHT algorithm is used to decompose spectrum and excitation for separate modification. Listening tests prove the proposed methods can effectively convert speaker's individuality while maintaining high speech quality with a small amount of training data.