The invention provides a method to real time monitor the ion beam. Initially, turn on an ion implanter which has a wafer holder, a Faraday cup and a measurement device positioned close to a special portion of a pre-determined ion beam path of the ion beam, wherein the Faraday cup is positioned downstream the wafer holder and the measurement device is positioned upstream the wafer holder. Then, measure a first ion beam current received by the Faraday cup and a second ion beam current received by the measurement device. By continuously measuring the first and second ion beam current, the ion beam is real-time monitored even the Faraday cup is at least partially blocked during the period of moving the wafer holder across the ion beam. Accordingly, the on-going implantation process and the operation of the implanter can be adjusted.