The gas-phase reaction of activation C-C double bond of C2H4 by CrO2+((2)A(1)/(4)A'') has been investigated using density functional theory (DFT) at the UB3LYP/6-311++G (3df, 3pd)//6-311G(2d, p) level. The calculation results show that the reaction experiences a rearranged process. On the basis of Hammond postulate, this is a typical 'two-state reactivity' (TSR) reaction. The involving crossing between the potential energy surfaces is discussed by means of the intrinsic reaction coordinate (IRC) approach used by Yoshizawa et al., and a crossing point (CP) is located, In addition, the orbital interaction analysis of activation C-C bond is carried out by fragment molecular orbital (FMO), and compared with the DCD model. (c) 2006 Elsevier B.V. All rights reserved.