Down's Syndrome subjects are known to possess certain speech quality characteristics besides their physical features. In this paper some of speech qualities are tested. By using X-ray technique the dimensions of the vocal tract for down and normal subjects are determined and used to get correlation between articulatory acoustic aspects. i.e.. the way" in which the formant frequencies (F1,F2 and F3; are related to the configuration of the vocal tract as a whole. This is achieved by using the mathematical tube models. Comparing the values with those obtained for the normal subjects , the reason for the characterized Down's Syndrome's speech is clarified.