Abstract: Based on the 3D thermal-hydraulics numerical simulation to the core outlet region of China Experimental Fast Reactor (CEFR) with the STAR-CD code, the rationality of the temperature monitoring-spot arrangement and the validity of the monitoring to blockage of assemblies were studied. The results indicate that the setting up location of the monitoring-spot is suitable and it is valid and large to diagnose blockage of assemblies by measurement of temperature in those spots.