Purpose: The purpose of this study was to explore the formant–articulation relationships in corner vowels by acoustic analysis of speech signals and measuring tongue contour using ultrasonography. Method: Eighteen healthy adults (8 men, 10 women), aged between 20 and 40 years, were enrolled. Speech signals of sustained vowels /ɑ/, /i/, and /u/ were recorded to obtain the first 2 formant frequencies. Tongue height, tongue advancement, and lengths of posterior tongue surface (LPTS) and anterior oral cavity (LAOC) were obtained using ultrasound image processing technique in order to examine the resonance mechanism of the oral vocal tract. Results: LPTS and LAOC, as well as mean frequencies of the first and second formants, showed significant differences between sexes and among vowels. The first formant significantly and better correlated with LPTS than with tongue height (r=78, p < .001, Pearson’s correlation). The correlation of the second formant with LAOC (r=85, p < .001) was also significant and better than the correlation with tongue advancement. Sex, however, did not show significant correlation with the formant frequencies. Conclusions: Ultrasonographic measurements of tongue contour correlated well to the formant frequencies. The results stressed the importance of tongue shaping in articulation. Although more studies are necessary in clinic alimplications, disorders associated with abnormal tongue shaping may be the target applications in the future.