The optimizations of air-condition systems using naturalrefrigerants R-600a are studied in this thesis. The theories including the exergy analysis, heat transfer and fluid mechanics are combined together to study the exergy transfer and destroy in each component. The optimizing parameters in this research include cooling air velocities, the tube diameters of evaporator and condenser. If all the conditions remain constant expect the tube diameter of evaporator, the numerical results display that the values of the total entropy generation rate with R-600a decrease from tube diameter 1.0 cm to 1.1 cm and increase from 1.1 cm to 1.2 cm. The tube diameter of evaporator and condenser at a minimum value of total entropy generation rate is 1.1cm for the simulation conditions. Besides, the coefficient of performance and the energy efficiency ratios also have maximum values at the tube diameter. If all the conditions remain constant expect the tube diameters of condenser, the tendencies of total entropy generation rate arethe same as those in evaporator.