Dinghushan national nature reserves is located in Zhaoqing, Guangdong Province, southern China(112°30′39″～112°33′41″E，23°09′21″～23°11′30″ N). Its total area is 1155hm2,with the altitude of 14.1～1000.3m. It is covered by tropical-subtropical forest, and in the South subtropical wetness monsoon climate areas. The relationship between α diversity of plant community with different sampling scales and environmental factors were studied in this paper. The permanent transect of plant communities was established in Dinghushan, from the upside of Dekeng to Sanbao apex, with the scale of 10m×1160m, that was subdivided into 120 contiguous 10m×10 m quadrats. It was 50.2m to 476.5m, with the absolute altitude 426m. Within each quadrat, altitude and other environmental factors were investigated, then every woody plant DBH≥1cm were identified into species, and tagged. The sampling scales were 10m, 20m, 40m, 80m and 160m, including the main vegetation types of Dinghushan: (1) monsoon evergreen broad-leaved considered as a zonal vegetation with a history of more than 400 years, (2) mixed coniferous broad-leaf forest, (3) evergreen broad-leaved in the valley, (4) Pinus massoniana forest. The analysis of correlation and principal component results were showed that different environmental factors had a different effect on α diversity in four plant crown layers: Tree, Shrub, Herb and Liana. The difference of α diversity was significant at different sampling scales. Simultaneity, it showed high environmental heterogeneity on transect. Therefore, it is reasonable to choose altitude grads as the main environmental grads to study the biodiversity of Dinghushan plant community. Altitude had closely correlation with α diversity on tree layer at all the sampling scales, showing that altitude was the most important environmental factor for α diversity on tree layer.