An ultra-wideband frequency response measurement system for optoelectronic devices has been established using the optical heterodyne detection method utilizing a tunable laser and wavelength-fixed distributed feedback-laser diode. By controlling the laser diode cavity length, the beat frequency is swept from DC to beyond 100 GHz, and ultra-wideband frequency response character can be measured without using any high-speed light modulation source and additional calibration. This is the major advantage of this method. In the measurement, two InGaAs p-i-n detectors have been tested, and their 3 dB bandwidths are 14.4 GHz and 40 GHz, respectively. Experiments show that this method is accurate and easy to carry out. Finally, several suggestions for improving measurement accuracy are given.