Safety integrity level (SIL) verification plays a critical role in reliability assessment of safety related systems. However, current methods available for SIL verification are too complicated to be applied in practice. Therefore, a novel method for SIL verification, which is based on system degradation using reliability block diagram (RBD), is proposed in this paper. The key idea of the method proposed is to perform RBD analysis and calculation of average probability of dangerous failure on demand (PFDG) at each stage of system degradation, which is caused by failures of redundant channels. The method has been applied to several classical redundant architectures of safety related systems, and could make the SIL verification process simpler. Further, the formulae obtained are identical with those given in IEC 61508.