This article is an analysis, from the phonemic point of view, of vowel alternation that occurs in bianyun in the Fuzhou Dialect. The author divides the tonemes of the dialect into the glottalized and unglottalized groups, and considers that the glottalization, originally being a device to distinguish the lower falling tones from the higher, has caused the vowels to become lower or to the back, thus resulting in the bianyun. He divides the bianyun into the phonetic bianyun and the phonemic bianyun, the former being only phonetic variations of vowels, while the latter having already evolved to morpho-phonemic alternations. The phonemic bianyun will be maintained even after the glottalized tonemes lose their glottalization and change to high tones. The bianyun in some Northern Mindong dialects would be an example. Finally, the author offers some critical remarks on the opinion that would treat bianyun as a case of lexical diffusion.