Abstract: The major types of oak forest—Quercus liaotungensis, Quercus aliena, Quercus dentata and Quercus variabilis forests in Beijing mountainous area are described. Oak forest is a kind of deciduous broadleaf forest consisted of temperate flora. The species of Compositae, Rosaceae, Graminae, Leguminosae are very rich. The plants playing an important role in plant community are Quercus spp. in tree layer, Spiraea, Prunus, Lespedeza, Vitex, Deutzia in shrub layer and Carex, Cleistogenes, Spodiopogon, Arundinella, Bothriochloa in herbaceous layer. The Hemicryptophyte is the most abundant among the various life forms, but Phanerophytes play the most important role: in the forest, The microphyll plants are very rich, the plants with mesophyll also play an important role. It is similar to Quercus dentata and Q. aliena forests. The Quercus variabilis forest is obviously different from Quercus liaotungensis forest among the oak forests.