An apparatus for monitoring an ion distribution of a wafer comprises a first sensor and a sensor. The first sensor, the second sensor and the wafer are placed in an effective range of a uniform ion implantation current profile. A controller determines the ion dose of each region of the wafer based upon the detected signal from the first sensor and the second sensor. In addition, the controller adjusts the scanning frequency of an ion beam or the movement speed of the wafer to achieve a uniform ion distribution on the wafer.