Abstract: Nuclear power plant is designed with multiple level of defense for the function used to protect the core and limit the spread of radioactivity during an event. The design of instrumentation and control system supports this multiple level design philosophy. Defense-in-depth and diversity is considered in the design of instrumentation and control system to maintain the integrity and availability of protective barriers or means and defense potential common-cause failures. This can effectively limit the development of accident, mitigate accident consequence, prevent the spread of radioactive material to the environment and ensure safety of reactor and plant equipment and staff.