Many people who suffer from infection of upper respiratory tract have advised to have tonsillectomy. The change of tonsillectomy may include that of range of voice, tone and resonance including range of voice quality after tonsillectomy. Nasal sounds are normally produced with an open velopharyngeal part during the production of nasal consonants and are described with nasality and nasalization. Nasality is one aspect of voice ""quality"" that has generally been assessed perceptually on the effect of surgery for maxillofacial disease and oropharyngeal disorders, and nasalization is considered as an acoustic phenomenon. In this present study, we analyze the change of formant frequencies for the sustained oral vowels, and the change of various parameters of nasality before and after tonsillectomy. As an assessment tool, we used computerized speech analysis systems, spectrogram and nasometer. On the spectrographic analysis, there were interesting findings that the frequency of first formant for vowel /i/ was decreased significantly after tonsillectomy, but the other frequencies of formants for vowel /a/ and /i/ were not changed after tonsillectomy. On the nasometric analysis, the nasometer is provided with objective measures of nasality derived from the ratio of acoustic energy output from the nasal and oral cavities of the speaker, there are interesting findings that the nasalance scores of sustained vowel were increased significantly after tonsillectomy, the slope scores of nasogram for nasal words were not changed and the nasalance scores for three types of nasometer passages were significantly increased after tonsillectomy.