In this work, we have studied the modification process of polystyrene surface by cold atmospheric pressure plasma jet to achieve a hydrophilic surface. For this purpose, dielectric barrier discharge plasma jet with different powers in short times with radio frequency power supply and argon plasma irradiation in atmospheric pressure were used. It was found that polystyrene surface changed from hydrophobic into hydrophilic after 20-s plasma irradiation and the wettability of surface increased with time. Surface evaluation was done by measuring the water contact angle of the samples before and after the modification. Fourier transform infrared spectroscopy showed the proof for the induction of oxygen-based functional groups in polystyrene when treated with the argon plasma. Plasma parameters were examined using optical emission spectroscopy, voltage-current (VI), and temperature measurements. The relation between plasma parameters and surface modification of the polymer is also discussed.