Although refrigerant maldistribution among parallel microchannel tubes is mainly caused by phase separation of vapor and liquid in the header, it is also affected by pressure drop in the header. This study experimentally investigates the pressure drop of single-phase and two-phase R134a flow in the vertical header of a multi-pass microchannel heat exchanger. R134a is circulated into the transparent header through multi-parallel microchannel tubes in the bottom pass and exits through multi-parallel microchannel tubes in the top pass representing the flow in the heat pump mode of a reversible system. The pressure drop in the vertical header causes the top tube has lower mass flow rate than the lower tubes for both single-phase and two-phase flow. The overall pressure drop in the header includes four components: acceleration, gravitation, friction, and minor pressure drop due to microchannel tube protrusion. The gravitation and minor pressure drops are dominant for two-phase flow at low qualities, whereas the acceleration and minor pressure drops are dominant for two-phase flow at high qualities. The acceleration pressure drop is negative because R134a exits through the branch microchannel tubes. It may cause the overall two-phase pressure drop in the header to be negative at high qualities. In this case, it is pressure gain instead of pressure drop at the top exiting part of the header.