Current transformers (CT) are used to measure the current in power transmission systems. Generally, the CT has an iron core. It is well known when the iron core saturates, the output signal of the CT is distorted in the commercial frequency. In short circuit testing for circuit breaker, when the prospective transient recovery voltage is measured, the CTs for high frequency measurement are often used to measure the interrupting current. However, it is not known the influence of the CT characteristics to the current measurements, for example di/dt and so on. In this paper, points to be noted at the current measurements in short circuit testing are discussed. Current was measured by shunt resistance and CT simultaneously. It was found that there is the difference of the shunt and CT output. Impedance and phase angle of high frequency CT were measured by frequency response analysis. Equivalent circuit of the CT is constructed from the results. The maximum value and di/dt of the CT output were smaller than the true current of the circuit by calculations using the equivalent circuit of the CT.