A low power inductively-coupled microwave plasma source based on planar spiral microstrip at 2.45GHz is presented in this paper.The resonant characteristics of the plasma source are analyzed with equivalent transformer coupled circuit model

and the discharges in various gas pressure based on the interrelation between absorbed power and plasma impedance are studied.The research shows that air plasma is ignited in low pressure as the input power is less than 220mW

and Ar plasma is ignited in atmospheric pressure as the input power is less 1.5W.The resonant frequency and S parameter of low power microwave plasma source change with excitation of microwave plasma.This may be a theoretical guidance for miniaturization of inductively-coupled microwave plasma source.