Powder diffraction patterns of ordered mesoporous materials are simulated with a newly developed program, which allows investigation of the influence of any desired matter distribution in the unit cell on the diffraction pattern. The simulation process can be subdivided into two major steps. First, a unit cell is generated from SiO₂ and, optionally, other building units. A weighted random placement of atoms is used to simulate the distribution of different atoms in different parts of the unit cell. This is done by a Fermi-type function, by which the probability of finding an atom on a site depends on the distance of a point from the center of a pore, leading to a smooth, continuous transition from wall to pore. Secondly, structure factors and then intensities of reflections are calculated, using the Lorentz correction and a geometric correction for powder data. The use of this program is demonstrated by the simulation of diffraction patterns, mainly for unmodified and modified SBA-15 as well as for MCM-41. Good agreement of simulated and experimental data is observed.