Radio fre quen cy cav ity is one of the most crit i cal and com pli cated com po nents in a cy clo tron. Dee volt age of radio fre quen cy cav ity ac cel er ates charged par ti cles to achieve re quired en ergy. Peak volt age of Dee is the key pa ram e ter of an radio fre quen cy cav ity. Bal anced Dee volt age is very im por tant for ef fec tive beam can ter ing and beam ex tract ing. An X-ray mea sure ment has been made to cal i brate and ver ify the peak volt age of Dee in a low-power (~20 kW) test. The X-ray mea sure ment for radio fre quen cy cav ity was de signed by means of brems strah lung. A suit able shield ing cover was cho sen for radio fre quen cy cav ity and the X-ray mea sure ment design was dem on strated ac cord ing to the the ory of pho ton trans mis sion. Fi nally, the peak voltage of Dee was ob tained at the power of 10-20 kW and the bal ance of Dee volt age was ver ified.