For no on-wafer measurement of photodetectors (PDs), a simple and effective method to determine high frequency electrical parameters of the equivalent circuit model is presented. Firstly, a coplanar waveguide (CPW) microstrip matched with the microwave probe is designed, and the measured output reflection coefficient shows good agreement with the theoretical design. The chip is mounted on the CPW microstrip and the output reflection coefficient of the PD test-fixture is measured. The equivalent circuit model including the PD, the bondwire and the CPW elements is simulated. By fitting the measured output reflection coefficient of the test-fixture, high frequency electrical parameters of the PD are extracted.