Abstract: The fuel assembly is the key part of a nuclear reactor. The China Advanced Research Reactor (CARR) standard fuel assembly consists of 21 fuel plates fixed to 2 side plates. The test objective is to research the hydraulic stability of the fuel assembly structure. Two CARR standard fuel assemblies were tested in the loop simulating CARR thermo-hydraulic conditions. The first assembly was tested for 60 days. It is found that the pin fixing lower comb damaged through the test, and severe damage of the comb as well as fuel plates are caused too. The second assembly was tested for 120 days after the improvement of the design and fabrication. The test shows that the assembly structure has good stability. At the same time, some important parameters were measured, such as the pressure difference of the fuel assembly, fuel plate vibration, the cladding surface corrosion depth, etc.