The vegetation of Sinharaja may be described either as a tropical lowland rainforest or tropical wet evergreen forest. Some striking characteristics of the forest are the loftiness of the dominant trees, the straightness of their bole, the abundance of regeneration and the diversity of species.
Average height of the trees varies between 35m – 40m while some rise even up to 50m. the vegetation of Sinharaja is that of humid wet evergreen forest type with a high degree of endemism. In fact some families such as Dipterocarpaceae show an endemism of more than 90%. The untapped genetic potential of Sinharaja flora is enormous. Out of the 211 woody trees and lianas so far identified within the reserve 139 (66%) are endemic.
Similarly, high levels of endemism are perhaps true for the lower plants like ferns and epiphytes. Out of 25 genera endemic to Sri Lanka 13 are represented in Sinharaja.
Each layer of the forest, the total vegetation density, including trees, shrubs, herbs and seedlings has been estimated to be around 240,000 individuals per hectare, of which 95% comprise individuals of the ground layer bellow 1m in height. The density of trees, lianas above 30 cm girth at breast height, ranges between 600 – 700 individuals per hectare while the number of merchantable individuals of trees of girth greater than 150 cm ranges between 45-55 individuals per hectare.