Kihansi Gorge forest is located within the Udzungwa Mountains in the Eastern Arc Mountains in Tanzania. The Gorge has critically endangered and endemic plant species, hence has national and global significance. Kihansi hydropower project started to be constructed in 1990 and its operation commenced in 1999. The construction of the hydropower facilitated the establishment of baseline study to monitor the forest. The main objective of this study was to monitor trees’ diversity using the established baseline study. Specifically this study intends (i) To assess changes of woody plant species composition and diversity for the period between 1997 and 2018 (ii) To assess changes in forest stand parameters between 1997 and 2018 (iii) To assess changes in tree species diversity and forest stand parameters along altitudinal gradients. Data were collected in 19 permanent sample plots established in year 1997 and 75 temporary sample plots installed in year 2018. The findings from statistical tests for all stand parameters for montane forest showed no significant differences (p > 0.05) throughout the study period. Basal area, volume and total carbon for miombo showed a significant difference (p < 0.05) for year 2006 and year 2009 and no significant difference (p > 0.05) for the year 2018. The study concludes that time to detect significant changes of forest stand parameters differ between vegetation types. Dynamic processes in the miombo woodlands are faster compared to montane forest. Based on altitude, tree species diversity in the Kihansi Gorge forest decreases as altitude increases. The study recommends monitoring to continue at Kihansi Gorge forest through the established permanent sample plots to ascertain the causes of the changes.Mufindi District Council