Speech quality is an interesting and very important aspect not only from linguistic and/or phonetic viewpoint but also from the viewpoint of speech technology. This study has been conducted from the latter point of view. A method has been developed based on the analysis-synthesis technique which enables to control voice quality by independently manipulating the voice source and the vocal tract resonant characteristics. Through this method, it is possible to investigate the amount of contribution of individual acoustic parameters to a certain voice quality including voice individuality. Formant frequencies and their bandwidths are used as the acoustic parameters to characterize the vocal tract configuration and the pitch frequency as the voice source. These acoustic parameters extracted from a natural speech are modified or changed to some extent and then a is synthesized making use of the modified acoustic parameters. Speech intelligibility and voice individuality are found to be controlled by this method. An application to a pathological voice has also been made to control the voice quality. It has been found that the method is capable of improving the so-called “roughness” or “hoarseness” of the pathological voice to a certain extent.