This paper presents an approach for the evaluation of software failures in software-based I&C (Instrumentation and Control) systems and/or equipment of NPPs (Nuclear Power Plants) based on Software Failure Mode and Effect Analysis (SFMEA). The presented approach is focused on application software of a generic software-based I&C system i.e. the part of the software of a programmable equipment of an I&C system which implements the I&C application specific functions (ICF). Generally, the ICF will be designed and generated by means of functional block diagrams of the underlying I&C system. The presented SFMEA approach considers the elementary function blocks (e.g. Boolean operators, delay modules, comparator modules) as the lowest abstraction level of the generic application software. The elementary function blocks are usually part of the engineering software of the I&C system platform and are implemented as individual software modules of a software library. The results obtained from the FMEA (Failure Mode and Effect Analysis) of elementary function blocks for an exemplary I&C system platform depict that the status of the input and output signals and the changeable parameters of elementary function blocks can contribute to software failure modes.