This paper argues that post-obstruent tensification of Modern Korean originated in the change of the phonemic interpretation for the same sounds: unvoiced and unaspirated obstruents. In 15th century Korean [s] was deleted optionally from the sequence [VsOV](O: obstruent interpreted as lenis phonemes), so that intervocalic obstruents(unvoiced and unaspirated) were then to be interpreted as fortis phonemes. Owing to optionality of s-deletion, [VsO1V] (O1 as lenis phoneme) varied with [VO2V](O2 as fortis phoneme). O1 and O2 were all the unvoiced and unaspirated obstruents so that O1 was interpreted as fortis phoneme after all. After syllable-final [s] became [t] in 16th century, the tendency which post-obstruent lenis consonants are interpreted as fortis phonemes stimulated post-[p] obstruents as well: [O] of word-initial consonant cluster [pO] was tensified in the first half of the 17th century and then so was word-medial [O] in the latter half of the 17th century. Finally post- [k] obstruents were tensified under the systematic pressure of phonemic sequence /obstruent-fortis/.