Abstract: Effects of size of gaps in a Cryptomeria fortunei plantation in the Zhougong Mountain, Southwest China on soil physicochemical propertis and species diversity were investigated. Soil samples and floral and faunal species were gathered from five gaps different in size, i. e. 50-100, >100-150, 200-250, 400-450 and 600-650 m², for analysis of soil properties, like pH, organic matter content, and some others and species diversity. Results show:(1) With the gap increasing in size, the number of species in various layers of the community exhibits a parabolic curve, that is, rising first and then falling, while Shannon-Wiener index (H), Simpson index (H') and Pielou index (J_sw) of the shrub layer and herb layer are higher in the gaps than under closed canopy; (2) The content of soil available potassium in the 0-20 cm soil layer is on the same trend; (3) In the gaps, variation of species diversity is related to a various extent to soil physico-chemical properties for instance species richness index (D), H and H' of the herb layer are closely related to soil pH, and D and H to soil available potassium (P<0.05), indicating that plant species diversity is closely related to soil pH and available potassium in the forest.