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Phou Khao Khouy National Park （PKK）, in the Lao People’s Democratic Republic, is rich in biological diversity, particular in plant species in the density forests. Knowledge of species composition provides basic information that is essential for protection and preservation of the existing forest and for sustainable forest management. The objective of this study was to explore species composition in plant community assemblage and of plant density, diversity and abundance in different vegetation strata. The study was conducted at Tad Xai in the PKK in2012. The total sample area comprised eleven50x50m plots and hierarchical plot design was employed to sample three vegetation strata （trees, sapling, and seedlings）. The measured indicators consisted of diversity, evenness and richness indices. Sixty-two families,145species and1,221individuals were recorded in the tree plots. The most abundant tree species in terms of individual number were Hydnocarpus ilicifolia, Hopea spp., and the most abundant families among trees in terms of species number were Rubiaceae and Dipterocarpaceae. Twenty-four families,33species and91individuals were recorded in the sapling plots, in which the most abundant species were Rinorea boissieui, Ailanthus triphysa and Hopea spp., and the most abundant families were Dipterocarpaceae and Myrtaceae. The seedling quadrate contained88families,153species and1,586individuals, the most abundant sapling species were Hopea spp., Hopea ferrea and Leguminosae spp., and the most abundant families among saplings were Rubiaceae, Annonaceae and Dipterocarpaceae. Diversity was extremely high at the trees and seedling levels, which showed diversity indices of2.63and2.83, respectively, but was low in sapling level （1.43）. The highest trees belonged to the Dipterocarpaceae, in which the mean height of17.24m and maximum mean DBH was33.57cm. The canopy cover of Diperocarpaceae was highest （35%） among families in the study area. Lauraceae and six additional families had lowest crown cover around5%.