A simple method to calculate the effectiveness of the control assembly in a fast neutron reactor is proposed. For each type of heterogeneous assembly (control or follower), a polar parameter, taking into account the assembly absorption and the axial leakage of neutrons inside the assembly, is defined. In a similar way, a bipolar parameter, taking into account the reaction of the assembly to a transverse flux gradient, is also defined. These two parameters, deduced from transport theory, are used to determine the absorption cross section and the diffusion coefficient of an equivalent homogeneous control or follower assembly. These new parameters are introduced in a one-group diffusion code, calculating the reactor as a whole with any number of control and follower assemblies. An approximate generalization to multigroup theory is proposed. Numerical comparisons show that this equivalent diffusion method gives results that are much closer to transport results than those obtained by the classical diffusion theory.