Surface modification of polycarbonate(PC) was performed to improve the wettability by Ar+ ion irradiation with 1 keV energy in oxygen environment. The ion dose ranged from 5 x 1014 to 5 x 1016 ions/cm2 and oxygen flow rate was also varied from 0 to 6 sccm(ml/min.). Contact angle was not much decreased from 78° to 48° for water and from 63° to 32° for formamide by Ar+ ion irradiation without oxygen gas, but largely reduced to 12° for water and to 8° for formamide as Ar+ ion irradiation with 4 seem oxygen gas. Surface energy of modified PC surface which was irradiated with oxygen gas was more increased than that of PC surface irradiated without oxygen gas. It is evident that the increase of surface energy for PC modified with oxygen gas is due to hydrophilic group which result from the chemical reaction between PC surface and oxygen gas. From X-ray photoelectron spectroscopy(XPS) analysis, the newly formed hydrophilic group is identified as hydrophilic C=0 bond, and atomic force microscope(AFM), it is found that the root mean square of surface roughness is changed from 14 Å to 22 ∼ 26 Å for Ar+ ion irradiation only and 26 ∼ 30 Å for Ar+ ion irradiation with 4 seem oxygen gas. Therefore wettability of PC surface is much more affected by newly formed hydrophilic group than surface roughness in keV energy Ar+ ion irradiation.