It is important that magnetohydrodynamic (MHD) flow velocity distribution in the cross section of a duct be related to materials compatibility, heat transfer, and MHD pressure drop. The first experimental results are given of the velocity distribution across the rectangular duct on the center plane and of the two-dimensional (2-D) MHD pressure drop effect due to the 2-D velocity distribution. The results show that both the boundary and core velocity distributions on the center plane of the duct increase with an increase of the Hartmann number M. However, the approach theory expected the core velocity distribution to decrease with an increase of M. The 2-D effect factor for the MHD pressure drop due to the 2-D velocity distribution was also carried out. This explains why the numerical results of the MHD pressure drop gradient are lower than in the experiments. Theoretical analysis of the 2-D and three-dimensional effects on the velocity distribution and MHD pressure drop is also included.