Core thermal hydraulics of the Superphénix reactor is investigated in the areas of the evaluation of core cooling conditions, thermal balances on core and subassemblies, values of main parameters during operation at power, and specific tests, such as control rod displacement, and their effects on power distribution and power flattening. These points are presented based on the results of calculation and the results obtained during commissioning tests. The margins to safety limits are given and compared to predicted values.