Mechanically polished polycrystalline copper was obliquely irradiated by an argon-ion beam with an ion energy of 1 keV. The incident angle and fluence of the argon ion were changed to obtain the dependences of incident angle and ion fluence on the surface flatness. After argon-ion irradiation, the surface flatness was examined using atomic force microscopy. Before the irradiation, the sample had a rough surface consisting of groove structures with an average width of 400 nm and an average elevation of 37 nm. The percentage of the surface area with a height more than 50 nm was 13%. The incident angle was changed from 0 degrees (perpendicular injection) to 70 degrees for the fluence of 1 x 10(18) Ar/cm(2). The fluence was changed in the range of 1 x 10(17)-1 x 10(18) Ar/cm(2) for the incident angle of 70 degrees. A smooth surface was obtained when the incident angle was largest, 70 degrees, and the fluence was greater than 5 x 10(17) Ar/cm(2). The percentage of the surface area with an elevation more than 50 nm was reduced to 2%, and the number of grooves per unit area was reduced to about half. The reduction of the elevation is mainly due to the selective sputtering for protruding parts, and the reduction of the groove number is mainly due to the redepositing of sputtered particles. The present results show that the oblique ion irradiation is useful for reducing both the elevation and the number of grooves of uneven surfaces. (c) 2006 American Vacuum Society.