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Microplasma can be found in many applications. The technology is also used for surface treatment of polymers. The process of surface treatment by microplasma is environmental friendly, and it could be realized at low cost due to the relatively low discharge voltage (around 1 kV) even under atmospheric pressure and the small size of the power supply and reactor. This paper presents the surface treatment by microplasma of polyethylene naphthalate (PEN) film using Ar gas and mixtures of Ar with N2 and O2. The microplasma process was analyzed by the emission spectroscopy method. An experimental Marx generator with MOSFET switches was used as a pulsed high-voltage power supply. The emission spectra were measured by an intensified charge coupled device camera, a spectrometer, and a photomultiplier tube. Surface wettability of the PEN film was confirmed by measuring the contact angle before and after the microplasma surface treatment. It was observed that the contact angle of the PEN film decreased, especially with the O2/Ar mixture. The analysis using an X-ray photoelectron spectrometer showed the decrease of C1s, which correspond to the C-H bond. The emission spectrum of the microplasma discharge in the N2/Ar mixture showed Ar peaks, OH peaks, N2 second positive band system peaks (N2 SPS), N2 first positive band system peaks ( N2 FPS), and decrease in the peak intensities of ArI and OH for microplasma discharge in the O2/Ar mixture. The lifetime emission signals for the N2 SPS for the microplasma discharge in 0.25% N2 in Ar were around 1 μs. The calculation of the microplasma rotational and electron temperatures shows low rotational and high electron temperatures.