This paper presents experimental results concerning the flow distribution of water cooling system in a rack. The parameters studied include the flow rate, manifold configuration and the length of header. The effect of gravity has also been investigated on the flow distribution. The experimental results of this research are summaried as follows: (1) The flow distribution of the U-type configuration is more uniform than that of the Z-type. (2) Gravity shows more pronounced effect on the uniformity of flow distribution when the outlet is located at lower position. For a higher inlet flow rate, the uniformity is worse. (3) The phenomenon of jet flow near the inlet does not necessarily increase non-uniformity of flow distribution.