This paper aims to investigate the acoustic characteristics of the vowels attested in Korean and French and to seek a way of understanding them from a phonological point of view. We first compare the two vowel systems by measuring the actual frequencies of the formants using CSL. It is shown that the first and second formants vary in wider range in French compared to Korean. In order to understand the two vowel systems from a phonological point of view, we apply the theory of Licensing Constraints, proposed and developed by Kaye (1994), and Charette and Kaye (1994). We propose the licensing constraints placed upon the vowels both in Korean and French. For Korean, we propose the licensing constraints such that both elements I and U must be heads. For French, we claim the following licensing constraints: U in a headed expression must be head, A cannot be head, and Nothing can only license an expression A in it.