The most important phenomenon observed in fibre suspensions is flow induced fibre orientation. Fibre size is the key parameter in prediction of fibre orientation which in turn describes the microstructures of injection moulded parts. This paper focuses on developing a three-dimensional numerical model for the analysis of fibre orientation prediction incorporating the effect of fibre size distribution using the finite volume method (FVM). The flow was considered to be incompressible, and behaves as non-Newtonian fluid flowing under non-isothermal condition. The hybrid closure model of Advani and Tucker was used to approximate the evolved fourth order orientation tensor. To validate the developed simulation model, several cases were modelled and compared with available experimental data for rectangular and cylindrical geometries. The simulation results showed that they are in good agreement with the experimental data. Hence, the numerical model could assist in decisions regarding the design of composite products. © 2014 Inderscience Enterprises Ltd.