If an assembly is partially blocked, the temperature in the upper plenum fluctuates at an early stage without a significant temperature increase. Therefore, the temperature fluctuation in the upper plenum can detect a partial blockage of an assembly. For developing the detection algorithm for a partial blockage, we numerically analyzed the temperature fluctuation in the upper plenum due to a partial blockage in an assembly. For analyzing the time dependent turbulence variables, the LES (Large Eddy Simulation) turbulence model in a commercial code was used. After analyzing the temperature fluctuations in the upper plenum, we studied the change of its characteristics according to the blocked conditions through some FFT analyses and statistical analyses. We found that the change of the skewness of the temperature fluctuation was the most significant parameter to detect a partial blockage and that its highest frequency is about 15Hz at 10cm beyond the exit of the assembly. Also, we have suggested that the resolution of the thermocouple should be less than 2 K in order to measure the fluctuated values of the temperature and that the response time of the thermocouple was less than 30ms.