Yttrium aluminum garnet (YAG) is a kind of excellent laser crystal, while it is a hard-brittle material with Mohs hardness of 8.5. In order to remove the defects such as scratches and pits easily produced during the polishing, copper plate was used as rough polishing and the effect of load and rotational speed on the flatness and surface roughness RMS was studied. Results show that copper plate can obviously improve the surface defects of YAG, and that the optimized parameters are rotation speed 70 r/min, load 15 kPa for 45 min. Then IC1000 was used to further reduce crystal surface roughness, and the best process are rotational speed 9 r/min and polishing pressure 15 kPa for 30 min polishing. At such condition, the flatness can reach around 100 nm and the surface roughness RMS can reach around 0.9 nm, with no scratches and pits observed under the stereo microscope. The proposed process could be a good reference for YAG polishing.