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Background and Objective: Parkinson's Disease (PD) is a progressive disease resulting from a reduction in the release of dopamine. This loss of dopamine can have an effect on non- speech movements, as well as various aspects of speech production. Dysarthria is a collective name for a group of speech disorders resulting from disturbance in muscular control over the speech mechanism due to damage to the central or peripheral nervous system. It produces problems in oral communication due to paralysis, weakness or incoordination of the speech musculature. A synthesis of research on normal and neurologically disordered speech should lead to an improved understanding of the neural regulation of speech. Subjects and Methods: In this cross-sectional study, 36 voice samples in normal voiceless environment were recorded. Frequency difference in high and low vowels was measured by Praat software. Data analysis was performed by Independent Samples Test and Maan-Whiteny tests. Results: All variables showed significant differences.F1-F0 in low vowels were significantly decreased in adults with Parkinson (P< 0.001). F2-F1 in high vowels was significantly decreased in adults with Parkinson (P < 0.05). Conclusion: The results showed that Parkinson disease affects formant frequency. [ABSTRACT FROM AUTHOR]