Abstract: It can be expressed as following equation for the change rule of the firstionization potential(Ip) of halohydrocarbonRX :Ip(eV)=3.3380+0.7344Ep(X)+2.7424(1/ax)qx-1.4302PEI where Ep(X) and ax are the ionization potential of outermost P electron and atomic polarizability for halogen atom respectively,qx is the partial charge on halogen atom X in halohydrocarbon molecule RX,and PEI is the polarizability effect index of alkyl group in the molecule RX.The result shown there were good agreement between the ionization potential values estimatedn by above expression and the experimental values for the investigated halohydrocarbon.