Abstract: This paper presents an experimental test system of CMOS microcontroller EE80C196KC20. Based on this system, the transient ionizing radiation effects on microcontroller were investigated using “Qiangguang-Ⅰ” accelerator. The gamma pulse width was 20 ns and the dose rate (for the Si atom) was in the range of 6.7×10⁶ to 2.0×10⁸ Gy/s in the experimental study. The disturbance and latchup effects were observed at different dose rate levels. Latchup threshold of the microcontroller was obtained. Disturbance interval and the system power supply current have a relationship with the dose rate level. The transient ionizing radiation induces photocurrent in the PN junctions that are inherent in CMOS circuits. The photocurrent is responsible for the electrical and functional degradation.