In order to smooth the uneven surface of polycrystalline copper, argon ion etching at the elevated temperature was conducted. The polycrystalline copper was obliquely irradiated by argon ion beam with ion energy of 1 keV and an incident angle of 70°. The substrate temperatures during argon ion irradiation were room temperature (RT), 473 K and 573 K. Before and after the irradiation, the surface morphology was observed using an atomic force microscope (AFM). After the irradiation at 473 K and 573 K, the surface was significantly smoothed compared with that at RT. In addition, the fluence required to obtain the smooth surface can be very low, compared with the case at RT. The present study shows that the oblique ion etching at elevated temperature is quite useful to obtain the smooth surface.