This system is the first system that allows to measurethe B1+/- field distribution in a real MRI system with a traceable absolute B-field calibration with a measurement uncertainty of better than ±1 dB. The relative measurement uncertainties in terms of the axial ratio and the relative phase of the B1x and B1y are better than 0.4 dB and 3%, respectively. Especially, for latest developments in MRI RF coil design employing multi-transmit systems the presented MRI B1 mapping system will become an indispensable tool for validation and quality assurance.