Natural fibres from regional plants like miscanthus grass have gained a lot attention as a replacement of synthetic fibres due to their eco-friendly nature, abundant local availability, low cost, reducing food versus fuel competition and their applications in bio-based composites. Nevertheless, the polar nature of natural fibres usually leads to a weak interfacial compatibility with hydrophobic thermoplastic matrices, resulting in poor strengths of the composites. In our study a way to prepare cellulose-based composites in a two-step extrusion process is investigated. The cellulosic fibres are first fibrillated from refined miscanthus fibres through (chemo-)thermomechanical pulping by extrusion. The fibre morphology including the aspect ratio is measured by field flow fractionation with optical measurement system. Infrared spectroscopy is applied to analyse the degree of substitution (DS) values quantitatively. In a second extrusion step, the modified cellulosic fibres are compounded with cellulose acetate (CA) and cellulose acetate butyrate (CAB) to cellulose-based injection-mouldable composites. The thermal, rheological, mechanical and morphological properties are characterised to analyse the composites. The properties of the composites prepared by this method are compared with their unmodified grades and with composites using drop-in biopolymers bio-based polypropylene (bio-PP) and bio-based polyethylene (bio-PE) as matrices. It shows, that the mechanical properties of mechanically fibrillated cellulose fibres reinforced CA are comparable to glass fibre reinforced polypropylene with similar fibre weight content. We have also seen, that the influence of mechanical fibrillation exceeds the influence of the chemical modification.