The purpose of this study was to investigate differences between native speakers' and non-native speakers' realization of stress-related duration and pitch patterns in English. Anderson-Hsieh et al. [Lang. Learn. 42, 529--555 (1992)] found that prosodic deviance showed a significant negative impact on native speakers' judgments of non-native pronunciation. Japanese uses pitch to highlight accented syllables; English uses pitch, duration, and loudness. Therefore, it is expected that Japanese speakers use pitch, when speaking English, more than American speakers, but use duration less. Experimental results confirmed this. Three Americans and three Japanese were instructed to read English sentences which included words containing six target vowels, in different stress conditions (reduced, unstressed, and stressed), sentence-medially and -finally. It was found that in American speakers' utterances, the difference in vowel duration showed constant increase from reduced to stressed. Japanese speakers' utterances also showed an increase, but the differences among the three stress levels were much smaller than native speakers'. The frequency of the pitch peaks showed the opposite pattern: Japanese speakers showed greater differences than the Americans. These results suggest that Japanese speakers realize English stress using the parameters for realizing pitch accent, leading to less acceptable English speech.