A practical method is derived for calculating the effective resonance cross sections in fast reactor assemblies with complicated geometry. The conventional equivalence relation between homogeneous and heterogeneous resonance integrals is extended to the complicated geometries which are composed of various types of plates, including two types of fuel plates with different compositions. This extended equivalence relation indicates that the effective cross sections in a heterogeneous system can be calculated by using a cross-section set of the Bondarenko type. The numerical results for the effective cross sections given by the present method are in good agreement with the exact values obtained by the RABID code.