[in Thai language with English abstract:] Study on important value and biodiversity index of lichens along vertical stratification of habitats on trunks of Castanopsis acuminatissima (Blume) A.DC. was performed at the lower montane rainforest at Khao Yai National Park. The objectives of this study were to use lichen community for environmental indicator and identifying status for conservation and utilization. Quadrates of 20 x 60 cm were placed at the base, mid trunk and canopy of five host trees. Thallus covers of lichens were estimated by drawing outlines of thalli and samples were collected for identification. Bark pH at the three levels were determined. The study found a total of 142 species. The highest important value (IV) was found in Dimerella sp.3, inhabited the tree base, and subsequently lower in, Phaeographina chlorocarpoides, the canopy inhabited, and Bacidia sp.1, the base occupied accounting for 0.165, 0.163 and 0.160, respectively. Biodiversity index show that diversity values, measuring richness of taxa, were 12.8, 20 and 23.6 from base, mid trunk and canopy levels, respectively. The diversity, measuring taxa that are unique to each ecosystem, of the three levels were 3.28, 3.3 and 3.09, respectively. The highest similarity of taxa was recorded from mid trunk and canopy accounting for 54.8%. Acidity of bark slightly declines along height with pH measured 3.5, 3.6 and 3.7 from base, mid trunk and canopy. All species were acidophyte and the majority were phototrophilic, epecialty those grow on canopies and mid trunks. KEYWORDS: Lichen, Diversity, Important value, Bark pH, Canopy