Variation in species diversity along ecological gradients is the main focus in recent biodiversity studies. Among multiple interacting resource gradients, altitude gradient may be a decisive factor determining spatial patterns of species diversity. Forest community classification, ordination, and species diversity patterns along altitudinal gradients can reveal relationships between species distribution and environmental factors, and provide an important basis for understanding the function of mountain ecosystems. For studying the species diversity along ecological gradients in karst ecosystems, we conducted a study in Mulun National Nature Reserve (MNNR), northwest of Huanjiang Maonan Autonomous County in Guangxi, China. This reserve and Maolan National Nature Reserve together possesses the largest and most typical karst forests in the world, and play an important role in plant biodiversity research due to their unique geography, climate, and vegetation conditions. The climax community in this region is an evergreen and deciduous broadleaf mixed forest on limestone in the central subtropical zone in MNNR. The area of this forest is 89.69 km² with 94.8% of vegetation coverage. The soil type is calcareous lithosols (limestone soil). Sixteen sample plots (20 m×20 m) were established in a typical depression landscape between karst hills on a southwestern slope in MNNR. All woody plants[DBH (diameter at breast height) ≥1 cm] from the 16 plots were investigated. We analyzed the pattern of woody plant species diversity along an altitudinal gradient in MNNR by measuring the indices of species richness, diversity, and evenness. The results showed that:(1) there were 5089 individuals in the studied plots, which belonged to 120 species, 48 families and 90 genera; (2) there was a distinct altitudinal pattern of woody plant species in the karst forest communities. Hierarchical cluster analysis and Non-metric Multidimensional Scaling analysis showed that the forest communities could be classified into three types, including those in lower slope, middle slope and upper slope respectively. In the lower slope, the dominant species were Cryptocarya austrokweichouensis and Cleidion bracteosum are; in the middle slope, the dominant species were Rubovietnamia aristata and Brassaiopsis glomerulata; Platycarya strobilacea and Rapanea neriifoliaare were the dominant species in the upper slope. As the altitude increases, the temperature and moisture also changed. Aridity and low temperature respectively limited plant growth at low and high altitude; the precipitation and sunshine were plentiful in middle elevations, which supports a high diversity of plant species; (3) among the woody species, there was a greater proportion of evergreen species than deciduous broadleaf species. In addition, coniferous species only grew on the top of the mountain; (4) Species richness and the Shannon-Wiener index, along the altitudinal gradient, exhibited a pattern of unimodal distribution, with the peak appearing in the mid slope; (5) the β diversity index fluctuated considerably, and decreased with rising elevation in the ecosystem. Our results will be useful for developing strategies of vegetation restoration in the karst region.