The composition and concentration of monoterpenes in the leaves of Artemisia japonica were determined. And seasonal variation in monoterpenes of A. japonica were investigated. Samples were taken from five sites at Mt. Muhak. Monoterpenes in the leaves were analyzed using gas chromatography-mass spectrometry. The leaf monoterpene fractions were dominated by R.T. 5.026, ${alpha}-thujene$, sabinene, ${eta}-pinene$, cyclohexane (R.T.=12.859) and R.T. 23.149. There were marked seasonal differences in concentration and composition of leaf monoterpenes. There were many kinds of monoterpenes in leaf of spring time, however most of monoterpenes concentration was small. On the other hand, there were not many kinds of monoterpenes in late time, but monoterpene concentrations were increased.