A systematic vegetation survey on the effects of forest gap size on vegetation diversity was conducted in a Cryptomeria fortunei plantation in Zhougong Mountain,Southwest China.Plots were explored in five different sizes of forest gaps,and closed canopy was selected as control.Dynamics of plant community composition and plant diversity in gap centre,gap edge,and closed canopy along the forest gap gradient were investigated.(1)A total of 141 vascular plant species belonging to 76 families and 113 genera were observed across 18 plots including 231 quadrats in this study area.Overall,species diversity shows an increase-decrease pattern with the increase of gap size.The species diversity in shrub layer hierarchically distributes as forest edge>closed canopy>gap centre,while herbaceous species as forest edge>gap centre>closed canopy.(2)The dominant species and important value varies among different sizes of forest gaps.C.fortune and Mallotus japonicus are the dominant species in small size forest gaps,with a sum of important value as high as 0.292 3.While Cunninghamia lanceolata and Betula luminifera are the dominant species in large forest gaps,with plenty of seedlings.(3)Species richness index (D),Shannon-Wienner index (H),and Pielou index (J_sw) reach relative high level in the large forest gaps with sizes of 400-450 m² for shrub communities,while these indexes reach high level in small forest gaps with sizes of 100-150 m² for herbaceous communities.Indexes of D and H among differing communities hierarchically show as forest edge>gap centre>closed canopy.These results suggested that forest gap can influences the composition of plant community and enhances plant diversity,and the large size of forest gap is more beneficial for species regeneration and plant diversity increase.