This study presents the work on process-mixture design of an experiment for wheat straw polypropylene. Fibre length and aspect ratio are two process variables that have significant impact on the mechanical properties of the composite material. Wheat straw was powdered to produce fibres, and those were fractionated into different fibre fractions to create grades with distinct fibre length and aspect ratio. These grades of fibres were used to evaluate the performance of those fractions as grades of straw fibre with respect to the performance in the final thermoplastic composites. The composite samples were made from each fibre grade at two different fibre loadings: 30 wt.% and 50 wt. %. One of the important findings of this work is the pattern of fibre size reduction during the compounding processes. These patterns can be used to correlate and optimize the utilization of different fibre sizes of straw fibre for different applications in thermoplastic composites. For example, fibres with initial width of about 360 mu m are the best fibres when a balance between modulus and impact properties is needed.