This paper aims to analyse the optional /j/ formation and /j/ insertion in Korean. t tends to provide the phonological interpretation of /j/ formation and /j/ insertion, considering the onset of the stem-final syllable, the number of syllable. In this paper, however, it is argued that speech rate also can influence these phenomena. For this purpose, I analyze speech of 10 speakers of standard Korean. Using the phonetic equipments and acoustic materials, I analyzed the phonetic values. As a result, this phonological difference is related to the onset of the stem-final syllable and the number of syllable. The possibility of /j/ formation is increasing when the number of stem-final syllable is 2. It is due to communicate effectively. One the other hand the possibility of /j/ insertion is increasing when the onset of the stem-final syllable is CV. This is because CV stem favour syllable structure like CjV to V stem in Korean. Then, I find out that the possibility of /j/ formation is increasing, when speech rate is high, while /j/ insertion is increasing, when speech rate is slow. Vowel-coalescence is increasing, on the average, when speech rate is fast. /j/ formation is increasing when speech rate is fast rather than normal and slow in that this is one of the Vowel-coalescences.