Polystyrene Petri dishes were modified by an ion-assisted reaction to supply a suitable surface for culturing cells. Wettability was measured by a contact anglometer after surface modification of polystyrene. Contact angles of water on the polystyrene were not reduced much by Ar+ ion irradiation only, but dropped rapidly to a minimum of 19 degrees, when polystyrene surface was modified by Ar+ ion irradiation with flowing oxygen gas. X-ray photoelectron spectroscopy analyses showed that hydrophilic groups were formed on the surface of polystyrene by a chemical reaction between unstable chains induced by the ion irradiation and the blown oxygen gas. Newly formed hydrophilic groups were identified as C-O, (C=O) and (C=O)-O- bonds. The influence of the ion beam modification in growth of the rat pheochromocytoma cells was investigated. The results showed exclusively preferential cell growth in the polystyrene Petri dish that was treated by the ion-assisted reaction.