Abstract: The structures and activities of 35 sesquiterpenoid antitumor agents are discussed, and found that each compound must have an active group with a conjugated π-bond. The values of their E_LUMO are imitatively calculated using CNDO method. It is evident that the smaller the value of the E_LUMO, the more active the original antitumor compounds, The differences between the values Of E_LUMO of the bases in DNA (and in RNA) and those of E_LUMO of the imitative sesquiterpenoids are so small that the charge can be transferred between them. It is postulated that this charge transfer complexing between the two electrophilic groups in sesquiterpenoids and the bases in double helix of DNA or/and RNA may contribute to the antitumor mechanism of this class of compounds.