This study was done to find out the difference in vowel length between the English language produced by American soldiers and KATUSA soldiers. Though English pronunciation by Koreans shows different features in many ways, I focused on English vowel length of KATUSA soldiers. I wanted to know if KATUSA soldiers, when compared with American soldiers, showed a foreign accent when they produced English diphthongs. The reason I wanted to deal with English diphthongs is that most Koreans have difficulty in pronouncing them since they do not understand the concept of diphthongs. Therefore I selected five English diphthongs, /aI/, cI/, /au/, /eI/, /ou,/as/ the experimental object, and I tried to find out the foreign accent of them. In this study I also tried to find out the vowel lengths in relation to their utterance positions. I investigated the difference of the English diphthong length between American and KATUSA soldiers using information gathered from experimental results.