The MHD pressure drop along a liquid metal NaK flow was measured around the junction of 3/4 in. and 1 1/2 in. o.d. circular 304-SS tubes under a transverse magnetic field in order to investigate the MHD effect of a sudden change of flow cross-section, since such a joint would be used in the design of MCF power reactors. The size and thickness of the tubes was chosen so that the wall-to-fluid electric conductance ratio C was almost the same between the large and small tubes. The experimental ranges: B = 0–1.5 T (Ha = 0–1400 for the smaller tube and Ha = 0–2800 for the larger tube), and Us = 0.2–2.4 m/s (Re = 3600–43 200).For the case of expanding flow, the pressure drops singularly in a distant location downstream from the junction and the singular location comes closer to the junction as the magnetic field is increased. The pressure drop between the two points just upstream and downstream of the junction is proportional to the mean flow velocity U times the square of magnetic flux density B. With increasing UB2, this pressure drop is reduced nearly to the sum of the MHD pressure drops predicted in the respective straight part of the small and large tubes. For the case of contracting flow, the pressure has a similar trend but scatters to a larger extent across the junction.