Abstract: The completion of k0 compilation for 112 (n, γ) reactions of NAA interest has made it possible to determine 63 elements parametrically. However, several problems are yet to be solved before all-round parameterization of reactor NAA can be achieved. In a series of four papers, starting from this one, some new suggestions are to be presented on parameterized corrections for both fission and threshold reaction interferences, and parameterized normalization of counting efficiencies for both "far" and "near" geometries.In this work, a "compound nuclear constant" , Ik0, is proposed based on the "1/v law" observance and small Q0 value of 235U (n, f) reaction. Ik0 values for eight major fission interference cases were determined in six irradiation channels belonging to three different kinds of reactors. The neutron spectrum-independent nature of Ika values verifies the reliability of Ik0 method. Interferences from (n, γ) reactions and reactor fast neutron induced (n, f) reactions on238U and232Th are also discussed.