Reliability is the most important factor in instrumentation and control (I&C) system for nuclear power plant (NPP). Now in order to reach high reliability, static redundant and dynamic redundant technique is in common use. Static redundant method can keep out the failure of one module, but could not detect the mistake. Dynamic redundant mean would examine the mistake of the working unit, but can not ward off the failure. In this study, our main purpose is to develop the advanced redundant way which includes the advantage of static redundancy and dynamic redundancy means by mixed redundant model (MRM). Utilizing logic order in the relationship between unit and system using reliable diagram, analyzing the reliability of MRM by the way of probability, the result has obvious effect upon improving the system reliability compared with above two models. MRM can be applied for safety critical I&C.© 2013 ASME