Abstract. Villanueva ELC, Obeña RDR, Origenes MG, Buot IE Jr. 2022. Vegetation structure of forests over limestone and its influencing environmental factors in Samar Island Natural Park, Philippines. Biodiversitas 23: 6492-6512. Samar Island National Park (SINP) forests over limestone, a nominee to the UNESCO World Natural Heritage List, needs a comprehensive study of its vegetation structure as the baseline for monitoring and effective conservation. Thus, an inventory was conducted in SINP inland forests over limestone with the following objectives: 1) to discuss the woody vegetation structure in the forests of SINP, and 2), to determine the environmental factors affecting the characteristic physiognomy and ecology of Paranas and Taft forests. Standard vegetation techniques in a total of eighteen 20m x 20m plots, classification and ordination analyses were conducted. A total of 3,578 tree individuals in 72 species, under 46 genera belonging to 35 families, were recorded from Paranas and Taft. Cluster analysis revealed 3 vegetation clusters named after the dominant species: 1) Shorea-Manilkara-Walaceodendron Cluster, 2) Shorea and Dracaena Cluster, and 3) Shorea-Manilkara-Hancea Cluster. Canonical correspondence analysis identified temperature as the most important environmental variable influencing the vegetation of SINP. Other environmental variables, despite their high rate of change and little effect on other species, may have multiple effects and indirect influences on other factors influencing the structure of vegetation in SINP. Thus, the development of appropriate conservation strategies is a must.