A CMP process is provided. A first polishing process on a wafer is performed using a first hard polishing pad with a first slurry. Then, a buffering process on the wafer is performed using a soft polishing pad with a cleaning agent to buffer the pH value in the first polishing process and to remove at least portion of the first slurry and the cleaning agent by the contact with the first soft polishing pad simultaneously. Thereafter, a second polishing process on the wafer is performed using a second hard polishing pad with a second slurry such that the pH value after the buffering process is between the pH value in the first polishing process and the pH value in the second polishing process. The method can avoid the scratch issue of wafer surface by particles resulting from pH shock and cross contamination.