We present in this paper a speech modification method for a person with dysarthria resulting from athetoid cerebral palsy. The movements of such speakers are limited by their athetoid symptoms, and their consonants are often unstable or unclear, which makes it difficult for them to communicate. In this paper, duration and spectral modification using Non-negative Temporal Decomposition (NTD) is applied to a dysarthric voice. F0 is also modified by using linear-transformation. In order to confirm the effectiveness of our method, objective and subjective tests were conducted, and we also investigated the relationship between the intelligibility and individuality of dysarthric speech.