The purpose of this paper is to investigate the distributions and acoustic characteristics of /ɨi/ found in Seoul Corpus. The monophthongization of /ɨi/ occurs in 93.04% of a total of 2,370 tokens and in higher than 99% of the cases with onsets and of non-initial position in a word. However, only 31.82% of word-initial tokens are monophthongized. 94.40% of monoph- thongized /ɨi/ tokens are realized as [i], but in word-initial position, 61.74% of the monophthongized diphthong tokens are realized as [ɨ]. Based on acoustic properties, /ɨi/ in Korean is not off-glide [ɨj] but on-glide [ɰi]. The 165 diphthongs which are realized [ɰi] are classified under two patterns. The first type (97 tokens) is composed of on-glide and vowel. The second type (68 tokens) is composed of only on-glide transition. Acoustically the first type is longer than the second type in duration. Also, the first type begins with a more front position than the second type and moves to a position that is more back than the second type.