Measurement of flow directed with respect to the magnetic field is performed using a directional Langnmir probe (DLP). A previously proposed practical formula for measurement of the Mach number under a weakly magnetized condition is modified for application to the directional flow field which consists of parallel and azimuthal flow components. The effect of density gradient caused by the rotational DLP measurement in the measurement of the radial profile of ion current angular distribution is modified. For the verification of the DLP measurement, parallel flow velocity is measured from the Doppler shift of Hell emission, while azimuthal flow velocity is calculated from E x B and diamagnetic drifts. It is revealed that this method provides plausible Mach numbers in the parallel and azimuthal directions with inclusion of the effect of density gradient. (C) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.