Abstract: An on-line test system of single chip microprocessor EE80C196KC20 for total ionizing dose radiation effects was presented. The total ionizing dose exposure was performed using a 60Co irradiator at a dose rate of 20 rad(Si)/s. The degradation process and sensitive parameters were investigated in detail. The failure dose threshold was obtained and the results show that the supply current and output voltage of I/O port and PWM change versus total ionizing dose regularly. The degradation mechanism was discussed associated with fabrication technology and circuit conformation and is significant for radiation hardness assurance.