Abstract: A new method bases on the second law of thermodynamics for entropy generation minimization analysis and thermodynamic optimization in the research and design of aircraft environmental control system (ECS). The chosen example is a military aircraft ECS with an air cycle machine (ACM) and a counter-flow heat exchanger. The effect of some design features for ECS entropy generation was discussed, such as the efficiency of heat exchanger and turbine in ACM, the air steam pressure and temperature of bleed from air-fighter engine compressor, and the air temperature of delivery to the cabin and comfort the pilot. Numerical results show that the temperature of the air stream that the ECS delivers to the cabin can be optimized for system operation at minimal entropy generation, when the components geometry parameters are fixed. The method illustrated is applicable to more complex and realistic aircraft environmental control systems.