Abstract: The response of ultra fast photoconduction detector made of SI-LEC GaAs crystal was studied experimently. The response time to picoseconds pulse laser and sensitivity to 532 nm direct current laser were obtained for GaAs detector. The experimental results show that the response time of the GaAs detector is about 100 ps and is independent of the bias voltage. However, it is dependent on the conditions of measurement system. The method of neutron irradiation and techniques change can improve the response time of the detector. The relationship between the sensitivity and the bias voltage is linear.