<p>Planarization of the REBa₂Cu₃O_y (RE: rare earth element) film surface is expected to improve the surface barrier against transverse magnetic field and enhance the asymmetricity of the critical current, i.e., the superconducting diode’s performance. The reactive ion etching (RIE), dry polishing with fixed abrasives, and wet polishing with loose abrasives are investigated as a surface treatment process for planarization of the REBa₂Cu₃O_y films. The RIE with various reactive gases, power, and treatment times resulted in localized etching pinholes on the film surface without change in flatness. In addition, the critical current was significantly reduced due to the degradation of the film during the RIE process. The dry polishing with fixed abrasives was simple and easy for planarization of the film surface. The root-mean-square roughness (RMS) value decreased from 10.5 nm of the untreated sample to 1.62 nm after the dry polishing process. However, numerous large scratches, which were caused in the process by coarse abrasive grains, blocked the current path and significantly reduced the critical current. The wet polishing with loose abrasives flattened the film surface to an RMS value of 0.82 nm without scratches. After the wet polishing process, the superconducting properties, such as the critical temperature and critical current, maintained the values they had before the process. The samples subjected to the wet polishing process show higher critical current asymmetricity at 77 K than the untreated sample possibly due to the enhanced surface barrier with the flatter surface. The critical current asymmetricity increases up to 145 % compared to the untreated sample with decreasing RMS value. The wet polishing with loose abrasives is a promising process for surface planarization of REBa₂Cu₃O_y film, and further surface planarization is expected to further enhance the asymmetricity of the critical current.</p>