Abstract. Setiawan AW, Tjiu A, Meididit A, Iswinanto, Ginanjar A, Atut Y, Agusti R. 2021. Plant diversity in logged over forest in Mahakam Ulu, East Kalimantan, Indonesia. Biodiversitas 22: 4829-4838. A study of flora was conducted in the Ratah Timber concession area. Objectives of this study was to calculate the Importance Value Index (IVI) and identify species diversity in Mahakam Ulu, East Kalimantan. A survey was carried out in 128 plots within 13 transects placed purposively. Vegetation sampling was conducted using line transect and a quadrat method. As many as 530 species belonging to 79 families were found. Of these species, 420 species were found in observation plots with 374 of the species being tree. The most dominant tree species were Endertia spectabilis, Shorea pinanga and Shorea leprosula with IVI of 10.24%, 8.25%, and 6.96%, respectively. In total, 114 species or 21.5% are endemic species of Borneo. The Shannon-Wiener diversity index at tree level was 5.51 and at seedling level was 4.95. The highest Similarity Index was found between transects 23 and 21, meanwhile the lowest was between transects 05 and 15. This study found 61 species which are classified on the International Union for Conservation of Nature (IUCN) Red List (2020-1) as critically endangered (16), endangered (11), and vulnerable (34) species.