An MR-TEM cell is a transverse electromagnetic (TEM) cell operated as a Tx/Rx coil directly inside an MR scanner. From a precise flip angle measurement in a tiny sphere of water the RF electric field inside the cell can be determined using the TEM condition |E|=2c0|B1+|. Thus, an MR-TEM cell can be utilized for the calibration of RF E- and H-field probes as well as for the determination of the RF voltages and RF currents at its ports which is important e.g. for experimental validation of simulation results in RF safety research. We report here on the high precision flip angle calibration of an MR-TEM cell with 0.1% uncertainty.