A method of determining a real time removal rate. A material layer is polished. During the polishing process, a light is incident onto the material layer continuously. The incident light is reflected from the material layer with a reflected light intensity. By integrating the reflected light intensity, followed by dividing the integration with a product of a differential of the reflected light intensity and the polishing time, an I-Dt transformation is obtained. The I-Dt transformation has a period which reflects the removal rate through calculation of optical principle.