A new method using a thick dielectric plate is discussed to measure the intensity distribution of a gyrotron's RF output in one step, unlike the conventional method in two steps. 1D simulation of the thermal diffusion in the plate removes the effect of convection cooling from the measured temperature distribution on the backside of the plate. As a result, the output radiation of UT-94 gyrotron is successfully measured. The peak intensity and the integrated power are 0.048 GW/m2 and 64.5 kW , respectively, within the uncertainty of 2.6%. The error of the measurement is expected to be less than 8.0 %.
