Based on trees with DBH≥1 cm in a 20 hm² stemmapped tropical rain forest in Xishuangbanna,we investigated spatial variations of Shannon and Simpson diversity indices across seven scales (grain sizes) (5 m×5 m,10 m×10 m,20 m×20 m,25 m×25 m,50 m×50 m,100 m×100 m,200 m×250 m).The results showed that:(1)Shannon and Simpson have the largest values at 100 m×100 m and 20 m×20 m,respectively.(2)The spatial variances of Shannon and Simpson indices generally decrease with scale,and with the smallest variances occurring at 100 m×100 m and 20 m×20 m,respectively.So we concluded that the reliable spatial scale for biodiversity estimation of Shannon diversity index is 100 m×100 m,whereas that of Simpson index is 20 m×20 m.(3)The variances of all the diversity indices are larger than those estimated by the random placement model,reflecting that spatial distribution of trees in the study area is more correspondents to habitat heterogeneity than random distribution.