A computational method, using the Flory-Huggins lattice model, the extended random copolymer theory, and a molecular modeling method to account for the end group effect on the chi interaction parameter and on the value of the temperature where the phase separation of a polymer blend occurs, is proposed. The overall chi interaction parameters are calculated based on the individual segmental interaction parameters of the end groups and the monomeric units in the polymer. The individual segmental interaction parameters are calculated from a combination of the Flory-Huggins lattice model and molecular simulation techniques. The chi interaction parameters of poly(dimethylsiloxane) (PDMS) and poly(methylphenylsiloxane) (PMPS) blends are calculated, and the results are compared with those from the experiment.(1,2) The end groups on PDMS and PMPS are alpha,omega-trimethylsilyl and alpha,omega-dimethylsilanol. The simulation results indicate that the end groups and their fractions in polymers at low molecular weights can significantly affect the chi interaction parameters and the phase separation temperatures. For example, in the case of blended polymers having 14 repeat units, the critical solution temperature of alpha,omega-trimethylsilyl-terminated PDMS and alpha,omega-dimethylsilanol-terminated PMPS is found to be 192 degrees C lower than that of alpha,omega-dimethylsilanol-terminated PDMS and alpha,omega-trimethylsilyl-terminated PMPS.