From the basic fraction of the ethereal extract of Ginseng Radix (Panax ginseng C. A. MEYER, Araliaceae), five methoxypyrazine and eight alkylpyrazine derivatives were identified by gas chromatography and gas chromatography mass spectrometry. The methoxypyrazine derivatives were likely to be of major significance in the characteristic earthy, green aroma with moldy undertone of Ginseng Radix. One of them was a novel methoxypyrazine derivative, 3-sec-butyl-2-methoxy-5-methylpyrazine (6), and the structures of 3-isopropyl-2-methoxy-5-methylpyrazine (3) and 3-sec-butyl-2-methoxy-5-methylpyrazine (6) were estimated from mass spectra of known methoxypyrazine derivatives, 2-isopropyl-3-methoxypyrazine (1) and 2-sec-butyl-3-methoxypyrazine (4), and confirmed by their syntheses. 3-sec-Butyl-2-methoxy-5-methylpyrazine (6) was found to possess a characteristic floral, moldy, green aroma with an odor threshold of 2.0 parts per 10^9 parts of water. The mass spectra of methoxypyrazine derivatives showed similar systematic fragmentations starting from McLafferty rearrangement and γ-cleavage, and the mass spectrometry was found to be a usefull method for structural elucidation and detection of methoxypyrazine derivatives.