This paper is to show the diphthong system of Jeju dialect speakers in their 70s or more on the basis of the acoustic analysis of their phonetic data. It is revealed through the analysis of their phonetic data that they clearly distinguish such diphthongs as [we], [w$epsilon$], [yc] and [yo]. However, this paper shows that they are phonetically insensitive to the separation between [ye] and [y$epsilon$] and they seldom make a precise pronunciation of diphthong [iy], which male speakers tend to pronounce to be [i] and female speakers to be [i].