The objectives of the present study were to identify composition of fungal communities associated with healthy and declined oak trees and seedlings in Ilam Province, Iran, and to evaluate their role in occurrence of the oak decline. Fungal isolates were obtained from branches of healthy and declining Persian oak trees and seedlings in Ilam province during summer and autumn 2014-2015. Fungal species were identified according to both morphological and molecular characteristics obtained from ITS, 28S and 18S regions of ribosomal DNA. Some fungal species such as Neoscytalidium dimidiatum and Obolarina persica were obtained only from branches of Persian oak trees with decline symptoms. The Acremonium sp., Coniochaeta sp., Cytospora ribis, Fusarium tricinctum, Microsphaeriopsis olivacea and Neoetophoma samarorum species were found only in healthy trees as endophytic species. While, B. mediterrana, Didymella glomerata, Fusarium solani and Tricothecium roseum were isolated from both healthy and declined trees. The F. tricinctum and T. roseum species were found in healthy seedlings. However, D. glomerata was isolated from both healthy and dried seedlings. The species B. mediterrana, D. glomerata, N. dimidiatum and O. persica showed pathogenicity on the Persian oak seedlings in the greenhouse conditions. Finally, it could be concluded that for the first time two species, D. glomerata and N. dimidiatum, were recorded as pathogens associated with Persian oak. In addition, Acremonium sp., Coniochaeta sp., C. ribis, F. solani, F. tricinctum, N. samarorum and T. roseum were recorded for the first time as endophytic fungi on Persian oak trees.