The aim of this study is to determine the effects of microclimatic factors and tree size on epiphytic lichen diversity in different parts of Quercus petraea. The microclimatic factors used were tree size, the base and trunk of the tree. bark surface moisture, and environmental factors such as the amount of relative humidity and light in the environment. A total of 35 epiphytic lichen species was found in the Quercus petraea forest in the Uludag Mountains. Parmelia sulcata was the most common species. The highest species diversity was found in small trees. Shannon diversity on the trunk of the tree decreased with the increase in the tree size. The relationship between tree size and species richness on the trunk of the tree was found to be more significant than that found on the base. It has been determined that the frequency of some species on the base and trunk of the tree was increased as a result of an increase in the amount of relative humidity and light in the environment and an increase in the bark surface moisture. At the same time the frequency decreased when the amount of relative humidity and light in the environment and bark surface moisture decreased.