This study experimentally and numerically investigates the single-phase flow into parallel flow heat exchangers with inlet and outlet rectangular headers having square cross section and 9 circular tubes. The effects of inlet flow condition, tube diameter header size, area ratio, flow directions (Z and U type), as well as the gravity are investigated. The experimental results indicate that flow distribution for U type flow is more uniform than Z type flow. Depending on the inlet volumetric flow rate, the flow ratio at the first several tubes can be lower than 50% of the last tube for Z type arrangement, and this phenomenon becomes more and more pronounced with the rising velocity at the intake conduit. The mal-distribution can be eased via reducing the branching tube size or increasing the entrance settling distance at the intake conduit. It is found that the influence of gravity on mal-distribution is negligible and the mal-distribution is associated with the jet flow pattern. The numerical result also shows a small eddy flow formed at the inlet of the first tube by the nearby vortex circulation which would reduce the flow rate to the front tubes.