This article examines the relationship between the speech proportion in the sentences of speaker and interpreter, and the accuracy of simultaneous interpretation from English into Korean. Audio tapes of simultaneous interpretations from TV broadcasts and international conferences by professional interpreters were analyzed with voice-editing computer software, and data were statistically processed. The results suggest that high sentence speech proportion in English had a negative effect on the accuracy of simultaneous interpretation; accuracy increased when the Korean interpreters maintained high sentence speech proportion in their simultaneous interpretation.