With technology scaling, radiation-induced soft error has been a major concern even for mainstream enterprise applications. Since various hardening solutions impose significant costs in performance, area and power consumption, full soft error protection can hardly satisfy the multiple design goals simultaneously. Recent studies have noted that the circuits have partial intrinsic immunity to soft errors. And the interest in using the circuit’s intrinsic immunity to improve the tradeoffs between reliability and various overhead from hardening is growing. However, most existing approaches cannot use it well. In this paper, the aim is to explore the principal source of the circuits’ immunity to guide fault tolerance to make better use of it. We categorized the components in a circuit into three classes: components with full immunity, with partial immunity and without immunity. By experimental and theoretical analysis, it is proved that the circuit’s intrinsic immunity mainly stems from the components with partial immunity.