To address the differences in total ionizing dose (TID) effect failure modes and failure doses of commercial microprocessors with varying feature sizes, an experimental study was conducted using microprocessors with 180 nm, 90 nm, and 40 nm feature sizes from the same manufacturer. A laboratory-developed, extendable online test system for assessing the TID effect of microprocessors was employed to monitor the status of communication, digital-to-analog signal conversion, non-volatile memory, random-access memory, direct memory access, consumption current, clock, and timers during the ⁶⁰Co irradiation experiment. The results indicate that the error doses for the three microprocessors are 331±36.28 Gy(Si), 355.5±41.51 Gy(Si), and 365.28 ± 20.15 Gy(Si), respectively. The failure models vary according to the feature sizes of the tested microprocessors. Specifically, the 180 nm microprocessor's most radiation-sensitive unit is the on-chip non-volatile memory, whereas for the 90 nm and 40 nm microprocessors, the device cores are the most radiation-sensitive units.