A practical formula for sensitivity coefficients of burnup properties has been derived by means of a generalized perturbation theory. A differential method is employed. The resultant formula corresponds to an extension of the Williams formula, which is based on a variational method. The applicability of the proposed formula is investigated by calculating sensitivity coefficients in a typical fast breeder reactor and by comparing the results with those from the direct subtraction calculations. The physical meanings of components of the coefficients are discussed.