This paper describes the calibration of Isolation Current Transformer by two methods, by conventional method and by two comparator based calibration method. A conventional method has limitations that we get fixed ratios and hence can go up to lower value of 1A. The uncertainties of the calibration system are in the order of 0.005 % for the ratio error and 0.01 crad for the phase displacement of the current transformer at 50 Hz. The power comparator based measurements can be done at test currents from 10 mA to 160A