The effect of solvent polarity on the initiation and propagation of ethylene polymerization with a constrained geometry catalyst (CGC) and MAO counterion was investigated by the density functional theory (DFT) with the conductor-like screening model (COSMO). Structures and energetics of reactant, pi-complex, transition state, gamma-agostic product, and beta-agostic product during insertion of ethylene monomer into the catalytic active species of ionic pair or cationic catalyst are analyzed using the software DMol(3). A comparative analysis on the energetics of the ethylene insertion in the gas state and in solvents with different dielectric constants (epsilon) shows that the solvent polarity affects the energetics of monomer insertion very differently depending upon the model for active catalytic species and/or polymerization stage. This different behavior provides a very useful method for identifying the real active catalytic species in experiment. The two important features of molecular structure determining the dependency of enegetics on the solvent polarity are the degree of exposure of the Ti atom on the solvent medium and the degree of separation of ionic pair during monomer insertion.