When a small ac voltage with two frequencies was biased to a probe in low pressure inductively coupled plasma, sideband current signals were observed. It was found that two frequencies of the small bias voltage are mutually modulated in the plasma, and this modulation results in the sideband current signals. Experiments for measurement of the sidebands were carried out at various pressures, correlations between the sidebands and the plasma state were investigated. The sidebands were not observed when the plasma was not generated: therefore these signals were produced by the nonlinearity of the sheath. The electron temperature could be obtained from the sideband signals, and it was in good agreement with that from a single Langmuir Probe. (C) 2011 Elsevier B.V. All rights reserved.