Increasing the significance of providing the software quality for high operation risk systems is concerned with the growth of the complexity and functionality of the safety important systems software and requirements to the reliability of these systems. The software verification and validation processes within the development and modification of systems of the automated control systems of nuclear power plants are of importance at the whole software life cycle duration In the paper, there are considered possible consequences concerned with violations of the technological process of the software development, ways of achieving the required software quality on the basis of the software verification and validation at all stages of the life cycle; approaches to the quality definition, procedures of software quality justification, models of the analysis of requirements to systems in the part of the software. The paper provides examples of the software quality evaluation over the indexes of ''Maintainability'' and ''Usability'' correspondingly for a safety important system of a nuclear power plant (NPP). All aspects of implementing works on the software quality analysis for NPP safety important systems in the part of the software quality evaluation are used under developing safety important systems of automated process control systems.