This paper aims at comparing the five English diphthongs, /ei/, /ou/, /au/, /ai/, /i/, in the aspect of length, to find out a common feature in /au/, ai/, /i/ and /ei/, and /ou/ and to see if there is any evidence between English low vowels and diphthongs. This study is a following research of Park (2009), and I analyzed each diphthong statistically to find out the corelation between English low vowels and English diphthongs, /au/, /ai/, /i/, beginning with English low vowels. To make this research, four Korean college students and five Americans participated in the experiment as subjects. All the subjects are female, and their age ranges from 21 to 39. Through statistical analysis of experimental data, I couldn`t find out anything in common between the length of the two English diphthongs /ei/ and /ou/. From this I could see that there isn`t any common feature between the two diphthong groups, which begin with front mid and back mid position, respectively. However, in the English diphthongs /ai/, /au/, and /i/, there is a common feature among them; the average length of the Korean subjects` pronunciation is less than that of the native subjects` pronunciation, regardless of its sentence positions. Because English diphthongs, /ai/, /au/, and /i/, have a common feature in that they begin with English low vowels, /a/ and //, I could get a hint that Koreans` difficulty of pronunciation of these English diphthongs may come from the difficulty of English low vowels, and we could use the results in the classroom in teaching English vowels effectively.