This paper studies factors that have influences on durations of syllables in Mandarn. Six factors are investigated by comparing means of duration in different categories in the preparatory study. The vector space is reduced from 299250 to 1000 by removing factors that do not cause significant changing in means and merging levels in each factor that have similar means. In order to remove influences from various initials and finals, durations of syllables are divided by the mean duration of the corresponding base syllables. The sum of squared residuals analysis is performed on the normalized duration. The results reveal that boundary index has the largest influence on syllable durations. Tone identity comes next. Though, position in word is not a very important single factor, it influences duration together with boundary index.