Abstract: China Experimental Fast Reactor (CEFR) is the first fast reactor in China. It experimented on CEFR to measure the ²³⁸U fission reaction rate in the first physics startup tests because of its fundamentality. The experiment was composed of the absolute measurement by the high poor uranium (²³⁵U-0.002%) and the indirect measurement by the national poor uranium (²³⁵U-0.335%) and the high enrichment uranium (²³⁵U-90%). The axial distribution and the radial distribution were given and compared with the theoretical value. The reliability was proved and a new way was found to measure the ²³⁸U fission reaction rate.