Both the current flowing through an antenna coil and the voltage supplied to the coil were measured for Ar inductive coupled plasmas (ICP) at a pressure of 13 Pa. By using the cancel circuit we measured the difference of the antenna coil currents between no plasma load and ICP. The measurement system was improved in order to reduce both radio frequency (RF) noises and waveform distortions. The impedance is presented when the cancel circuit is used. A simple method to obtain the value of the coupling coefficient is presented. The current measured with the cancel circuit could be related to the plasma current much better than that without the cancel circuit. The values of the coupling coefficient increased as the RF power increased, and ranged between 0.3 and 0.5 in our experiment.