This paper proposes a new application of speech modification called ''speech morphing''. In image processing, morphing is a well known technique that gradually changes one person's face to that of someone else. Speech morphing produces duces similar results for speech; i.e., one person's speech is gradually changed to that of someone else, Speech morphing makes it possible to create movies or multi-media entertainment together with image morphing. The proposed algorithm pitch-synchronously modifies fundamental frequency (F-0) and DFT spectrum and outputs high quality speech. To clarify the balance of F-0 modification and spectrum modification, listening tests were carried out using 20 male speakers. The results yielded the relationship between the amount of modification and speaker identity. In terms of overall performance, listening tests show that the proposed algorithm successfully generates smooth, high quality voice changes.