This study investigated the chemical composition of Petasites japonicus (S. et Z.) Maxim essential oil. During the period 2011~2013, P. japonicus (S. et Z.) Maxim plant was investigated for composition of the essential oil. Chemical composition and characteristic compounds of the essential oils from the aerial parts of the plant according to the crop year studied. The essential oils consisted of sesquiterpene compounds, which were the most abundant components. Samples collected in 2011 were found to be richer in oxygenated sesquiterpenes, while samples collected in 2012 and 2013 were richer in diterpene alcohols and sesquiterpene hydrocarbons, respectively. Ninety-two compounds were identified in the P. japonicus (S. et Z.) Maxim essential oil of 2011, and caryophyllene oxide (20.49%), β-caryophyllene (10.28%), β-bisabolene (6.80%), and alloaromadendrene (6.50%) were the major compounds. Seventy-four compounds were identified in the plant essential oil of 2012, and phytol (17.22%), α-farnesene (15.31%), α-caryophyllene (9.93%), and β-caryophyllene (6.12%) were the major compounds. Ninety-two compounds were identified in the plant essential oil of 2013, and α-farnesene (22.42%), α- caryophyllene (21.49%), pentadecane (15.35%), and germacrene (5.70%) were the major compounds. The content of most of the chemical constituents varied significantly with different harvesting time. The content of α-caryophyllene and caryophyllene oxide was increased significantly from 2011 to 2013. The content of α-caryophyllene and isocaryophyllene was decreased significantly from 2011 to 2013.