Background & Aims: Previous studies have indicated significant differences in vocal parameters between children with Down syndrome and normal children, but there are challenges about these differences. In this study vocal parameters and Maximum Phonation Time (MPT) in adults with Down syndrome have been investigated. Method: This cross-sectional and analytic study was performed on 22 adults with Down syndrome and 22 normal adults matched for age and gender. While subjects were asked to produce sustained vowel /a/, some vocal parameters such as Jitter (perturbations of frequency), Shimmer (perturbations of amplitude), and Fundamental frequency (F0) were analyzed by Vocal analyzer of Dr. Speech (version 4.3 U from Tiger Electronics) Software. Mean MPT was also determined in both groups and based on gender by applying usual method of producing sustained vowel /a/ as much as possible. Two-way analysis of variance was performed for data analysis. Results: In Down syndrome group mean F0 was higher than Control group, but Jitter was lower (P<0.001). However, difference of mean shimmer between two groups was not significant. Interestingly in both groups shimmer and jitter in females were lower than males. In regard to MPT, two groups did not differ significantly. Conclusion: Although the differences between Down syndrome and control group show physiological lack of laryngeal control, it may be resulted from other factors (e.g. personal characteristics and submissiveness in social interactions) not investigated in the present study.