Woody vegetative coverage of Zagros forests is mainly composed by oaks. Information on natural patterns of genetic variation is of fundamental practical importance for sustainable forest conservation and management. In the present study, to assess the genetic differentiation and population diversity of eight Quercus brantii populations with 104 genotypes from Ilam forests, genomic DNA was extracted and then amplified fragments from different primers were visualized and scored on 1.2% agarose gel. Of 22 screened ISSR primers, 15 primers produced repeatable and scorable bands. Dice and Jaccard similarity matrix were analysed. Then the data were molecularly analyzed using molecular analysis software, GenAlEx. The selected primers amplified 189 bands, from which all were polymorphic. The number of amplified fragments varied from 7 to 20 with an average of 12.6 bands per primer, and with size range of 100 to 1800 bp. Analysis of molecular variance (AMOVA) showed that major proportion of total genetic variation (79%) was attributed to within populations, and the remaining 21% was resulted from among population variation. High level of genetic variation was observed within Q. brantii natural populations from Ilam forests.