A micro-discharge air plasma jet was developed operated at low frequency (50 Hz) ac high voltage (kV). The micro-discharge plasma jet was generated by feeding air through a dielectric installed between two disk-shaped electrodes with a hole of 1 mm diameter in the center. Both of the electrodes were made of copper. The micro-discharge in the dielectric is evolved as a plasma jet from the outer electrode through its hole. The ac power supply was a neon light transformer. Optical emission spectroscopy was employed to study the plasma discharge characteristics. Specair software was used to analyze the collected spectrum. The reduced electric field and electron density of plasma as well as rotational and vibrational temperatures, referring to ion or gas and electron temperature respectively were measured by processing emission spectrum.