In electronegative or reactive plasmas, the problems such as negative ions floating near the sheath edge or deposition contamination cause more challenges for the diagnosis of conventional Langmiur probe. The electron density measured by microwave resonance probe is only a function of dielectric constant of plasma, there should be less or no influence of electronegative or reactive plasma. In this paper, a floating microwave resonator probe is proposed to measure electron density of capacitively coupled Ar plasma. A comparison with Langmuir double probe measurement shows that microwave resonance probe is applicable for measuring low electron density of plasma. The experimental results from the measurements of Ar/SF6 and SF6/O2 capacitively discharge driven by 40.68 MHz show that addition of SF6 into Ar plasma reduces the electron density significantly, with further increase of SF6 flow rate, electron density shows a gradual decrease. While for the addition ofO2 into SF6 discharge, the electron density continuously decreases with the increase ofO2 flow rate. Additionally, the electron density does not vary with lower frequency input power for SF6/O2 capacitively discharge driven by 40.68 MHz/13.56 MHz. The preliminary interpretations of the above experimental phenomena are presented.