For reducing the liquid metal MHD pressure drop of the first wall cooling, a NaK experimental study was made on the effects of the electrical insulation of a rectangular duct. Three inner surfaces of a 2.1 mm thick 304-SS rectangular duct of 20.5 mm × 45.5 mm inner cross-section was coated by 1.3 mm thick FRP plates, remaining one of the 45.5 mm wide faces uninsulated to simulate the plasma facing first wall. The magnetic field was mainly applied in parallel to the uninsulated face. The results are summarized as follows. (1) The MHD pressure drop gradient is proportional to the mean flow velocity U and also to the magnetic flux density B. (2) It is about 2.3 times higher than the value predicted by Shercliff's theory for a completely insulated rectangular duct. (3) It is largely reduced, for the same velocity, down to 7.7% at B= 1.0 T and 5.1% at B= 1.5 T in comparison with the uninsulated duct. These results are encouraging for applying to fusion power reactors.