This paper describes an approach tovoice characteristics conversion for HMM-based text-to-speech synthesis system using speaker interpolation. An HMM interpolation technique is derived from a probabilistic distance measure for HMMs, and used to synthesize speech with untrained speaker's characteristics by interpolating HMM parameters among some representative speakers' HMM sets. The results of subjective experiments show that we can gradually change the characteristics of synthesized speech from one's to the other's by changing the interpolation ratio.