An apparatus for detecting an endpoint of a grinding process includes a connecting device, a timer and a controller. The connecting device is connected to a sensor that periodically senses an interface of a reconstructed wafer comprising a plurality of dies of at least two types to generate a thickness signal comprising thicknesses from a surface of an insulating layer of the reconstructed wafer to the interface of the reconstructed wafer. The timer is configured to generate a clock signal having a plurality of pulses with a time interval. The controller is coupled to the sensor and the timer, and configured to filter the thickness signal according to the clock signal to output a thickness extremum among the thicknesses in the thickness signal within each time interval, wherein the thickness signal after the filtering is used to determine the endpoint of the grinding process being performed on the reconstructed wafer.