<p>This work aims to determine the effect of cellulose fibre content and fibre size and shape on the crystallisation and solidification of polylactic acid (PLA) during cooling to better understand microstructure development in biocomposites. Hot melt twin-screw extrusion and injection moulding was used to prepare cellulose fibre reinforced PLA composites with different microstructures from the diluted to the concentrated regime. Differential scanning calorimetry, temperature-controlled optical microscopy and rheometry were used to study the non-isothermal and isothermal crystallisation and solidification of the composites. The non-isothermal analysis shows that cellulose fibres, regardless of their content, size and shape, did not affect either the PLA thermal characteristics (glass transition, cold crystallisation and melting temperatures), nor its degree of crystallinity. On the other hand, isothermal crystallisation kinetics of PLA is greatly influenced by the fibres, being accelerated when fibre content and length/aspect ratio are increased. Our results also reveal that crystallisation and related rheological behaviour during solidification of PLA can be tuned by modifying the fibre characteristics, thus contributing to better control PLA processing, especially when studying thermoplastic processes involving isothermal steps.</p>