The purpose of the study is to analyse and classify the characteristics of Gongsanseong forest through phytosociological and quadrat method with 20 plot samples. In result, the forest was classified as Quercus acutissima community. The subcommunity was composed of Quercus aliena and Zelkova serrata. The average vegetation coverage of Quercus aliena subcommunity was upper tree layer (83%), lower tree layer (48%), shrub layer (39%), and herb layer (30%), while the average vegetation coverage of Zelkova serrata subcommunity was upper tree layer (76%), lower tree layer (52%), shrub layer (40%), and herb layer (45%). The order of importance value of the forest community with DBH 2cm above plants was Quercus acutissima (72.62), Quercus aliena (40.52), Prunus sargentii (19.81), Styrax japonica (19.39), Zelkova serrata (15.78), Robonia pseudoacacia (14.76), Quercus variabilis (13.83), Sorbus alnifolia (13.71), Platycarya strobilacea (10.74), Pinus densiflora (10.08), and Quercus serrata (9.31). According to breast diameter analysis results, it is expected that the importance value of Prunus sargentii and Zelkova serrata will be continuously increased while Quercus sp. is dominating the forest. With the result of ordination analysis, the relationship between the forest community and environmental factors could be affected by soil nutrition, moisture, and elevation etc.