The development of a new composite that is compounded of natural fibres and of a low price polymer, such as HDPE or PP, began in the last decade of the past century. While this is a rather new material no attempts have been made to analytically describe and simulate mechanical properties of this material. There is also a great lack of knowledge in describing fine tuned processing parameters. Therefore, in the first part of this article micromechanical approach based upon Generalised Method of Cells (GMC) is introduced to simulate properties of injection moulded wood-plastic composite compounded of polypropylene (PP) or polystyrene (PS) and of wood or cellulose short fibres. Materials have first been scanned with an optical and electron microscope to determine average fibre properties and their scatter. These values are then used to determine elastic and plastic response of the composite alongside with its tensile strength and maximum elongation, where the Tsai-Hill failure criterion has been used. The results of the simulation are then compared to experimental data in order to evaluate practical usage of this method. The second part of the article is focused on the simulation of injection moulding where test specimens were injected and the search for optimum injection parameters was performed. Various mechanical and rheological tests were performed and in addition practical industrial products were injected to observe the impact of various natural fibres on the filling of the mould cavity. Every experiment and process was then compared to the numerical simulation in order to evaluate applicability of numerical simulations under real conditions. On the basis of these experiments detailed mould design guidelines are given.