Mathematical modelling of physical and mechanical properties of plastic foam as well as numerous practical applications requires knowledge of foam structural characteristics. A necessity exists to determine the characteristics of the spatial structure of inhomogeneous materials comprising inclusions of other material, e.g., polyurethane foam without destructing the material and analysis of each element. A methodology is elaborated for preparing highly porous plastic foam specimens and investigation of foam strut-like structure with light microscopy (LM) by taking images in three mutually perpendicular planes. A mathematical model is developed for highly porous plastic foam for the determination of probability density functions of its building elements—polymeric struts': (a) length and (b) angles, using LM images in three mutually perpendicular planes. Computer codes are created and parameters of distribution functions for strut's length and angles are calculated using experimental data for verification. A good correspondence of the modelling results with experimental data is proved to exist. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39477.