Geometrical optics and the Monte Carlo method are very flexible in dealing with the interaction of light with non-spherical particles, but usually diffraction is not considered. To cover this gap, the Heisenberg Uncertainty Monte Carlo (HUMC) model is applied to calculate separately the diffraction of a ray or a photon. In this paper, we report an improvement of the HUMC model by specifying the phase of the photon subject to the Fraunhofer diffraction condition. After validating the model by comparing its results with analytical results for apertures of simple shapes, the HUMC model is then applied in simulations of Fraunhofer diffraction by apertures of complex shapes, such as those composed of one or two elliptical openings. We have shown that the diffracted intensity distributions of simple apertures obtained by the HUMC model are in good agreement with the results calculated from analytical expressions. The simulations of diffraction by apertures composed of two square or elliptical openings prove that the HUMC model is a powerful and flexible tool for predicting the Fraunhofer diffraction by a complex optical system. (C) 2015 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.