In this study, the characteristic properties and plasma parameters of capacitive radio frequency (RF) argon (Ar) discharge and supplementary discharge at low pressure are investigated with optical emission spectroscopy (OES). The wavelengths of spectral lines from OES are obtained between 650-900 nm. Using OES lines and related experimental data, the electron temperatures for different RF power, flow, and measurement periods are determined. Eventually, the properties of plasma including the electron temperature are identified. In addition, the numerical modelling of the discharge parameters is carried out by using the "extended" fluid model for plasma, and consistency between numerical and experimental results is discussed.