Purpose: The first real‐time‐MRI‐guided radiotherapy system has been installed in a clinic and it is being evaluated. Presence of magnetic field (MF) during radiation output calibration may have implications on ionization measurements and there is a possibility that standard calibration protocols may not be suitable for dose measurements for such devices. In this study, we evaluated whether a standard calibration protocol (AAPM‐ TG‐51) is appropriate for absolute dose measurement in presence of MF. Methods: Treatment delivery of the ViewRay (VR) system is via three 15,000Ci Cobalt‐60 heads positioned 120‐degrees apart and all calibration measurements were done in the presence of 0.35T MF. Two ADCL‐ calibrated ionization‐chambers (Exradin A12, A16) were used for TG‐51 calibration. Chambers were positioned at 5‐cm depth, (SSD=105cm: VR's isocenter), and the MLC leaves were shaped to a 10.5cm × 10.5 cm field size. Percent‐depth‐dose (PDD) measurements were performed for 5 and 10 cm depths. Individual output of each head was measured using the AAPM‐ TG51 protocol. Calibration accuracy for each head was subsequently verified by Radiological Physics Center (RPC) TLD measurements. Results: Measured ion‐recombination (Pion) and polarity (Ppol) correction factors were less‐than 1.002 and 1.006, respectively. Measured PDDs agreed with BJR‐25 within ±0.2%. Maximum dose rates for the reference field size at VR's isocenter for heads 1, 2 and 3 were 1.445±0.005, 1.446±0.107, 1.431±0.006 Gy/minute, respectively. Our calibrations agreed with RPC‐ TLD measurements within ±1.3%, ±2.6% and ±2.0% for treatment‐heads 1, 2 and 3, respectively. At the time of calibration, mean activity of the Co‐60 sources was 10,800Ci±0.1%. Conclusions: This study shows that the TG‐ 51 calibration is feasible in the presence of 0.35T MF and the measurement agreement is within the range of results obtainable for conventional treatment machines. Drs. Green, Goddu, and Mutic served as scientific consultants for ViewRay, Inc. Dr. Mutic is on the clinical focus group for ViewRay, Inc., and his spouse holds shares in ViewRay, Inc.