This study focuses on the effects of cellulose content and cellulose morphology on the structural and mechanical properties of PP/cellulose fibre composites. A detailed structural characterisation was performed, including SEM-BEI analysis of fibre spatial distribution and X-μCT analysis of fibre orientation. The results indicated that the fibres were well-dispersed and oriented mostly along the main direction of the PP/cellulose test specimen. Based on light microscopy it was found that the cellulose fibres acted as nucleators in the PP matrix. By use of DSC it was shown that addition of cellulose fibres increased both the crystallinity and T_c. The increased crystallinity is mainly a result of the transcrystallinity formation. The tensile properties of the composites were affected by the fibre dimensions and the fibre concentration. The long fibres yielded the highest increase of Young's modulus. Elongation at break was mostly affected by the fibre concentration. The results thus indicate that there is a major potential for improving the mechanical properties of fibre-reinforced composites, provided an adequate concentration of fibres is applied.