Risk caused by safety-critical I&C systems considerably affects overall plant risk. Software failures in digitalized I&C systems must be considered as the cause of risk. As digitalization of safety-critical systems progresses, the need for software failure probability quantification increases. For the software of safety-critical systems, very high reliability is required. This article aims at providing an overview of promising software failure probability quantification models for this kind of safety-critical system: The software reliability growth model (SRGM), the input-domain-based test model (IDBT), and the validation/verification quality model (VVQM). In order to accommodate the characteristics of safety-critical systems, a more effective framework of practical risk assessment applications is necessary. In this article, we propose the combined use of SRGM&VVQM for a more systematic and traceable method of the failure probability quantification of safety-critical software.