Abstract: We studied changes in floristic composition, community structure and tree species diversity along an altitudinal gradient on Mt. Mang (eastern Nanling Mountains) and Mt. Mao′er (western Nanling Mountains). Floristic composition, but not vegetation type, differed greatly between these two mountains. Maximum tree height decreased sharply with increasing altitude on both mountains. On Mt. Mao′er, maximum DBH, basal area and stem density peaked at intermediate elevations. On Mt. Mang, maximum DBH and basal area showed significantly negative correlations with altitude while stem density did not change significantly with altitude. These unimodal patterns or negative relationships of DBH, basal area and stem density might result from altitudinal gradient at the meso-scale and soil properties, microtopography, and human disturbances at the small scale. Tree species richness on Mt. Mang was obviously higher than that of Mt. Mao′er in the evergreen broad-leaved forests below 1000 m and in brushwoods above 1700 m. On both mountains, species number correlated negatively with altitude, however, this trend was insignificant on Mt. Mang due mainly to human activities at high altitudes. Alpha diversity of tree layer kept stable with increasing altitude and did not differ significantly between these two mountains. Considering tree species composition and diversity of evergreen broad-leaved forests on these two mountains, species richness (S) and α diversity (H′) of the evergreen broad-leaved forests of Mt. Mao′er were lower than those of Mt. Mang. This might stem from human activity and the invasion of many deciduous broad-leaved species and bamboo (Phyllostachys pubescens) on Mt. Mao′er.