Abstract: 【Objective】Determine the short-term influence of forest-gap size on plant diversity in a Platycladus orientalis plantation.【Methods】This study investigated plant diversity within three sizes of 2-year-old forest gaps(“S”, “M” and “L”, with radius of 4 m, 8 m, and 12 m, respectively). 【Results】① Species diversity values were higher in all forest-gap plot sizes than those in the control plots. Relative to the control plots, species number increased in the shrub layer by 4,7, and 2. In the herb layer by 4, 1, and 3, and in the canopy layer by 2,1, and 1, in the L, M, and S gaps, respectively. ② With the exception of the shrub layer in the S gap size, diversity values, as measured by the Margalef index, were significantly higher in all forest-gap sizes than those in the control plots. The shrub layer had the highest diversity in the M gaps and the herb layer had the highest diversity in the L gaps. The Shannon-Weiner index showed that species diversity in the shrub layer was significantly greater in the L and M gaps than that in the control plots. The Pielou uniformity index indicated that species evenness in the herb layer was significantly lower in the L gaps than that in the control plots and had a slightly negative relationship with forest-gap size. 【Conclusion】Plant diversity and tree species regeneration in a P. orientalis plantation in Xuzhou City were higher in the M forest gaps than they were in the S and L gaps.