The heat and flow analyses of a parallel-flow heat exchanger are performed. Two models with and without considering the effects of the geometric characteristic of flat tube are used. Comparing the two models, the modeling using the heat transfer correlations of flat tubes shows the better accuracy and stability of numerical solutions. The effect of flow distribution on the thermal performance is examined with varying the design factors (i.e., the locations of separators and inlet/outlet, and the aspect ratios of microchannels of the heat exchanger). The flow uniformities along the paths of the heat exchanger are proposed, and are observed to evaluate the thermal performance of the heat exchanger. The optimization using the ALM method has been accomplished by maximizing the flow uniformity. It is found that the heat transfer rate of the optimized model is increased by 6.0% compared to that of the base. type and the pressure drop by 0.4%.