— To improve the irradiation accuracy in hadron therapy, a lot of studies are in the evaluation uncluding how to integrate MRI and PET at the treatment of cancer patients. These two future techniques will offer simultaneously the monitoring of the beam during the irradiation. In this study and for the first time, we calculated the dose distribution in a head volume for several carbon ion energies and at different values of the magnetic field B using fluka MC code. Afterwards, either longitudinal or radial dose deflection was simulated with and without B . Basically, the Z axis is entered by the beam and the radial deflections mean the ones corresponded to the X axis which has the same direction of B . The maximum longitudinal deviation of bragg pick depth was 0.672 mm at 200 MeV and the maximum radial deflection was almost 4.23 cm at 100 MeV. Both of them are in the presence of B = 1.5 T. Our results are in a good agreement with previous experimental results (a maximum relative error founded of 2.08%). The results of this study will offer the monitoring of the dose deposition with good accuracy in the presence of a magnetic field in carbon ion therapy.