Plant species composition and forest structure of a lowland rain forest fragment (42 ha) was studied. Within 40 plots (400 m2), all mature trees and saplings were surveyed; seedlings were surveyed in subplots (20 m2 area). 2031 mature trees of 157 species (12 unidentified) belonging to 45 families were recorded. Tree density (678 ha− 1) was high compared to other studies, but the basal area (27 m2 ha− 1) was moderate compared to other secondary forests or logged forests. Biodiversity indices were fairly high (Shannon's index = 2.91; Simpson's index = 0.95). Scaphium lychnophorum, Endospermum chinense, and Artocarpus rigidus were dominant in the upperstory layer (15–25 m), and Gironniera subaequalis in the understory (5–15 m). There were significant correlations between physical factors (terrain, soils, and elevation), floristic composition, and forest structure. Moreover, the aboveground biomass of the forest was affected by disturbances, especially past logging activities.