The potential energy surfaces of the gas-phase unimolecular decomposition reactions of 1-chloropropane and 2-chloropropane are investigated by various quantum chemical methods including CCSD(T), CCSD, GBS-QB3, C3B3, MP4, MP2, and B3LYP. Modified strong collision/RRKM theory was used to calculate the unimolecular rate constants as a function of pressure and temperature. It is found that the major reaction pathway is the HCl elimination. It is predicted that bond dissociation reactions have negligible contribution to the overall rate constant. The computed rate constants are compared with the available experimental data.