To provide accurate effective cross sections for core calculations, the multiband method was applied to light water reactor assembly calculations. The multiband method has been extended to arbitrary geometries by introducing band-dependent currents at the boundaries of a region. The transport of neutron is treated by the angular space-dependent current coupling collision probability method. A fuel assembly is divided into heterogeneous domains where the multiband method is applied directly by using collision probabilities. Several examples of numerical calculations for UO2 and mixed oxide fuel assemblies are shown. The space dependence of the effective cross section can be expressed accurately by this method, which leads to an accurate prediction of k∞ values.