This study investigates whether the shortening of duration caused by fast speech rate and syllable final stop has an effect on the spectral characteristics of the diphthongs in Hangzhou Chinese (HC). Diphthongs in the open and closed syllables produced in normal and fast speech are investigated. The results indicate while the duration of the diphthongs are shortened in fast speech as compared to normal speech, the spectral targets of the diphthongs are not substantially changed, which may be due to the increased articulatory effort. The diphthongs in the open and closed syllables are different in temporal and spectral characteristics, which are not likely to be a direct result of duration shortening. The relationship between vowel duration and spectral reduction is discussed in the paper.