Abstract: The total ionizing dose effect of high energy (1.5 MeV) electron irradiation on the DC characteristic of Si bipolar junction transistor (BJT) was investigated. The total fluence of electron ranged from 5×10¹³ cm^-2 to 1.2×10¹⁶ cm^-2. It is shown that the offset voltage (V offset) emerges after the BJTs are irradiated, and the V offset increases with the total dose rise. The V offset of BJT is caused by the irradiation-introduced defect levels that work in two ways. On one hand, the reduction of carriers caused by the levels eventually induces that the variation of build in potential in BC junction is bigger than that in BE junction. On the other hand, the defect levels result in that the conduction band variation of BC junction is bigger than that of BE junction.