The development of vowel articulation was investigated in 103 patients with Down's syndrome without hearing impairment, ranging from 4 to 21 years of age. As a control, 88 normal children ranging from 4 to 15 years of age were examined. Five Japanese vowels were recorded and the lowest three formant frequencies (F1F2F3) were estimated by use of a sound spectrograph. The formant frequencies were plotted on the Iri's vowel chart by using the following formulae. a=10log (F2/3√F1F2F3) b=10log(F3/3√F1F2F3) For the group with Down's syndrome, organic conditions of the articulatory organs, the intelligence levels and the sentence structures in conversation were also examined. At the age of 4 to 5, Iri's vowel charts showed a complete separation of the areas of five vowels in normal children, while the areas overlapped considerably in patients with Down's syndrome. With reference to their mental age (MA), however, the vowel charts of the patients with Down's syndrome showed a separation of the front vowel at MA 3 years and a complete separation of five vowels at MA 5 years. This order of the vowel differentiation in terms of the mental age for the patients with Down's syndrome was similar to that of the normal children reported by other authors. In the patients with Down's syndrome, mental development seems to be closely correlated to the development of the vowel articulation, as well as to that in the sentence structure in conversation.