As a part of a series of studies now under way, this paper discusses the analytical aspect of the problems encountered in the application of multilevel formalism to the fast reactor Doppler effect analysis in the unresolved region. The concept of the “statistical collision matrix” introduced by Moldauer1 was used. The paper is divided into two parts. Part I describes the formulation and statistical consideration of the problem. For S-matrix formulation, the Doppler broadened cross sections using ideal gas model can be expressed in terms of the well-known broadened line shape functions. These functions are readily amenable for reactor calculations using any existing resonance integral code with some trivial modifications. The statistical behavior of the S-matrix parameters is also discussed in some detail. In order to improve understanding of the nature of the problem, an illustrative example was carried out analytically for the case of two interfering levels. Two more realistic examples pertinent to the fissile isotopes of interest are also given by numerical calculations using 50 interfering levels. These examples provide good qualitative descriptions of the statistical behavior of the S-matrix parameters that one may expect in the reactor Doppler effect studies. Part II deals with the application of the multilevel formalism in the Doppler effect studies.