This study investigates the chemical compositions of Aster koraiensis Nakai essential oils and the quantitative changes of major terpene compounds according to various harvesting times. The essential oils obtained by hydrodistillation from the aerial parts of Aster koraiensis Nakai are being analyzed by GC and GC-MS. The essential oil compositions of Aster koraiensis Nakai are characterized byhigher contents of sesquiterpene compounds. Ninety-seven volatile flavor compounds are being identified in the essential oils from Aster koraiensis Nakai harvested in 2010, and caryophyllene oxide (8.38%), aristolene (7.08%), epiglobulol (5.57%), and ethyl furanone (4.73%) are the most abundant compounds. Ninety-five compounds are identified in the essential oils from the plants harvested in 2011, and aristolene (11.56%), calarene (9.33%), phytol (8.28%), ethyl furanone (7.63%), and epiglobulol (7.18%) are the most abundant compounds. Ninety-five compounds are being identified in the essential oils from the plants harvested in 2012, and calarene (15.3%), aristolene (14.24%), ethyl furanone (7.21%), phytol (6.98%) are the major compounds. The contents of aristolene, ${alpha}$-caryophyllene, ${alpha}$-muurolene, ${alpha}$-calacorene, aromadendrene oxide, and calarene increase significantly from 2010 to 2012. The contents of ${alpha}$-cubebene, isocaryophyllene, and diepi-${alpha}$-cedrene epoxide decrease significantly from 2010 to 2012. The quantitative changes of aristolene and calarene according to harvesting time can be served as a quality index of the Aster koraiensis Nakai essential oils.