Theoretical computations and analyses are made of the efficiencies of self-sustained discharge transverse-flow CO2 lasers. A model of one-dimensional steady flow, quasi-two dimensional discharge and three vibrational relaxing modes is used under various conditions of discharge, flow and cavity position in the flow direction. Efficiencies of the cascade processes (input electrical energy-energy of upper vibrational levels, energy of upper vibrational levels-output laser energy, and input electrical energy-output laser energy, are given as frictions of electrode width, gas-fiow velocity and cavity position, A theoritioal reference is thus provided for properly selecting laser parameters, such as discharge conditions, flow parameters and cavity configurations.