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Nitrogen plasma treatment of a polystyrene layer in a vacuum chamber resulted in changes in its hydrophobicity properties. The purpose of this work was to investigate the effect of chamber pressure on nitrogen plasma treatment on the hydrophobicity properties of polystyrene surfaces. The plasma character was diagnosed by optical emission spectroscopy (OES) determining the plasma parameters, i.e. electron temperature (Te) and the electron density (Ne). Boltzmann plot method, ratio, and Stark Broadening were utilized to calculate the parameters. The hydrophobicity properties of polystyrene surfaces were measured by contact angle measurements, and polar groups studied with FTIR. The plasma diagnostic results showed that higher the chamber pressure resulted in greater intensity of the emission spectrum. The spectrum exhibits the highest intensity at the wavelength of 385.715 nm, indicating the existence of NII or N2+ ion species. The increase of the chamber pressure declined the electron temperature but increased the electron density. Results of the FTIR measurements indicated the presence of the new CH3 groups that are polar groups causing the surface to change characteristic, from hydrophobic to hydrophilic.