Abstract: In this paper, modulation of the space charge in a magnetically insulated transmission line oscillator (MILO) caused by the radio-frequency field (including the radiation wave field and the space charge wave field) is researched at first. Formula of the first order component of the harmonic current at saturation (nonlinear regime) is obtained. When the space charge wave is considered, it will enhance the harmonic current on condition that the signal is small and the radiation wave is in phase with the space charge wave, but the case is not certain with a large signal. The harmonic current will enhance with the increasing of the voltage on the diode and reduce with the increasing of the operating frequency and the saturation length.