Abstract: Reactor neutron irradiation on phototransistor was investigated. Collector current of phototransistor decreased after exposure to neutron fluence from 3×10¹¹ to 5×10¹²cm^-2, which was attributed to the degradation of gain and photoresponsivity. The reciprocal of gain was linear with fluence and the slope of the line decreased with injected current increasing. Theoretical analysis shows that heavily doping base region or shortening base width can enhance radiation hardness of gain. The primary photocurrents under different reverse-biased voltages are basically equal before irradiation and gradually differ when neutron fluence increases, and the degradation is small for the high reverse-biased voltage. Radiation tolerance of primary photocurrent can be improved by applying PIN structure or increasing reverse-biased voltage which broaden depletion-region and lower the proportion of diffusion current which is very sensitive to radiation. The dark current of phototransistor decreases with fluence increaseing, which is different from that for PIN photodiode.