The present paper investigated the insulation characteristics of gas mixtures containing CF3I as an alternative to SF6 from the point of view of electron swarm parameters. The density-normalized effective ionization coefficients and electron drift velocities are calculated for the gas mixtures of CF3I with Xe, He, N2 and CO2, by solving the Boltzmann equation in the condition of steadystate Townsend (SST) experiment. The overall range of the density-normalized electric field E/N is from 200 to 800 Td (1 Td = 1017Vcm2). From the variation curve of the effective ionization coefficients with the CF3I mixture ratio, the limiting field strength for which the ionization exactly balances the electron attachment is determined. The calculated results are valid for theoretical analysis of the insulation performance of CF3I and its gas mixtures as replacement of SF6. Additionally, the global warming potential (GWP) is also taken into account to evaluate the possibility of applying in gas insulation of power equipment.