R-410A upward flow in a transparent vertical header of microchannel heat exchanger was investigated experimentally. Refrigerant was provided into the transparent header by five tubes in the bottom pass and exits through the five tubes in the top pass representing the flow in the heat pump mode of reversible systems. Visualization revealed that the flow morphology in the header, affected by the inlet quality and mass flow rate, was very important to the refrigerant distribution and heat exchanger performance. A microchannel evaporator model incorporating the experimental results was developed to evaluate the evaporator performance. For the single pass heat exchanger, the numerical results showed that the capacity degradation was up to 40% compared to the uniform refrigeration distribution. The capacity degradation was related to the inlet conditions and header geometry through the derived empirical correlation. The maldistribution problem is more significant in the two-pass heat exchanger. Its capacity is lower than that of the single pass heat exchanger. Thus, the single pass heat exchanger has better performance.