[en] A program module for three-dimensional calculation of geometry of reactors and critical assemblies with complicated structure is described. The module is intended for work with the HMKFK program complex. An algorithm for calculation of a particle trajectory in a given geometrical zone is considered. Three subroutines comprised in the module: the DGXYZ subruotine for initial data input, the SSXYZ subruotine for grawing the initial coordinates of a particle series from the given distribution, and the GEOXYZ subroutine for for constructing a particle trajectory are described. The GEOXYZ subroutine of the module is the basic one; it is multiply referred to for the construction of a particle trajectory portion between two collisions. Before reference to the GEOXYZ subroutine, the following data should be given: particle coordinates and direction of flight; optical length of a particle free path between two collisions; particle energy (or energy group number). The module programs are written in the FORTRAN language. The module described enlarges the library of standard modules incorporated in the MMKFK program complex