In this study, an acoustic emission (AE) sensor is used to monitor the hard polishing process of a sapphire wafer. Since the AE sensor is very sensitive, a specially designed polishing head is used such that the AE sensor can be placed as close to the polishing area as possible. Then, hard polishing experiments are conducted to investigate the effects of hard polishing processing factors including pressure and rotational speed on the material removal rate (MRR) and root mean square (RMS) value of AE signals. According to the results, it is shown that the RMS value of AE signals strongly correlates with the MRR of the processes under various polishing conditions. On the basis of the experimental results, a mathematical model of MRR is developed. The coefficient of determination (R2) of the model is about 99%, the corrected coefficient of determination () is about 95%, and the prediction error is less than 12% in all cases studied. The results indicate that the AE sensor is a potential tool for monitoring the polishing process.