Abstract: In this paper, the total ionize dose (TID) irradiations for NPN bipolar transistors were carried out by 60Co γ rays. Obvious degradations were observed after irradiation, which are manifested as the increase of base current and the decrease of current gain. A more obvious increase of base current was observed at lower emitter junction bias. It shows that the degradation of the base current mainly origins from the increase of the recombination current of the emitter junction, since the recombination current component in base current is dominant at lower emitter junction bias. It can be concluded that the performance degradations of bipolar transistors are mainly due to the increase of recombination current caused by radiationinduced traps in the oxide. It is found that the bias condition during irradiation is the key factor affecting the TID effect of NPN transistor. The reverse bias is a worse bias condition than the zero bias during irradiation. Furthermore, the TID effect of two kinds of NPN bipolar transistors was compared. The two kinds of transistors have the same structure, except that one of the devices is radiation hardened by improving the surface state of the base region. The radiation induced degradations in radiation hardened devices are less than that in unhardened devices. For the unhardened device, the base current increases from 8.82×10-8 A to 3.67×10-7 A at |VEB|=0.5 V after 50 krad(Si) irradiation, increased by 316%. While for the hardened device, the base current increases from 7.51×10-8 A to 1.19×10-7 A at |VEB|=0.5 V after 50 krad(Si) irradiation, increased by 58.4%. Finally, the deep level traps in NPN transistors were measured by deep level transient spectrum (DLTS) before and after irradiation. The radiation will increase the trap densities and alter the energy level of the traps for the unhardened devices. By comparing the DLTS results of hardened and unhardened devices, it is found that there are great differences in their native traps. The energy level of the traps in the hardened devices is farther away from the center of the band gap than the unhardened devices. It may implies that the radiation hardness of the device can be achieved by improving the native trap states of the device.