Although Standard Korean is based on Seoul Korean, the phonetic realization of syllable codas in Seoul Korean has not been satisfactorily investigated. This paper aims to study how Seoul speakers pronounce syllable codas in certain phonetic contexts and what pronunciation they prefer among variants. It is noted that the realization of a syllable coda is different word by word and generation by generation. It is also noted that the syllable coda of a word is realized differently depending on the following vowel. And we discussed how the Principles of Standard Korean Pronunciation should be revised in the future, based on the results of this study.