[en] The radial distribution of BATAN CS-30 cyclotron magnetic field has been observed and measured. The observation is perfomed in order to determine wether the magnetic field distribution meets the relativistic distribution that will maitain the stability od accelerated particle orbit. The measured values from innermost radius to the outhermost radius were compared to the theoritical calculated values for proton. it was found that the magnetic field radial distribution did not fully agreed with the relativistic distribution. The maximum phase difference accumulation of moving proton and electrode potential was found to be 66.3 degrees. However, it was considered that it should not cause decleration effect on particle. (authors). 4 refs.; 3 figs