Species composition, frequency distribution of diameter classes, species diversity, and stem vitality of woody plants were analyzed in a Mongolian oak (Quercus mongolica Fisch. ex Ledeb.) forests in permanent quadrates of Mt. Nam and Mt. Jeombong, which were installed for Long Term Ecological Research (LTER). The principal objective of this study was to clarify the ecological characteristics of both sites by comparing the Mongolian oak communities established in Mt. Nam surrounded by urban area and in Mt. Jeombong as a natural area, to accumulate the basic data for long-term monitoring, and furthermore to predict possible changes in vegetation due to climate change. The species composition of the Mongolian oak community on Mt. Nam differed from that of Mt. Jeombong. Such differences were usually due to Sorbus alnifolia, Styrax japonicus, Oplismenus undulatifolius, Ageratina altissima and so on, which appeared in higher coverage in Mt. Nam. Species diversity of the Mongolian oak community in Mt. Nam was lower than that in Mt. Jeombong. This result was attributed to the fact that the Mongolian oak community in Mt. Nam is under continuous management and was dominated excessively by S. alnifolia, and S. japonicus, which were originated from artificial interference and chronic air pollution. As the results of analyses on the frequency distribution of diameter classes of major tree species and the transitional probability model based on Markov chain theory, the Mongolian oak community in Mt. Nam showed a possibility of being replaced by a S. alnifolia. Considering that this replacement species is not only a sub-tree but is also shade-intolerant, such a successional trend could be interpreted as a sort of retrogressive succession. The Mongolian oak community established in Mt. Jeombong differed from the community in Mt. Nam in terms of its probability of being continuously maintained.