Composite materials from polystyrene and lignocellulosic hemp strands, comprising 15% by weight of cane straw, were prepared by injection moulding. In the first step, the mechanical properties of untreated hemp strands were studied. The mechanical properties of composites reinforced with untreated hemp strands showed acceptable behaviour in comparison with those obtained by glass fibre composites. The mechanical properties at yield of both kinds of composites were very similar, with differences lower than 10%. The same tendency was observed for the specific mechanical properties. In a second step, the hemp strands were submitted to a refining process (physical modification) and, later on, the refined hemp strands were chemically treated with alkyl ketene dimmer (AKD) in order to improve the mechanical anchoring and the adhesion with the matrix. Both treatments increased the mechanical properties of the hemp strand composites. The evaluation methods used were colloidal titration, optical microscopy, scanning electron microscopy, image analysis, Fourier transform infrared spectroscopy, and moisture absorption and floatability test.