Abstract: The effects of wood fibre (WF) size on isothermal and non-isothermal crystallization morphology and capacity of PLA in WF/PLA composites were investigated with differerntial scanning calorimetry (DSC) and polarising optical microscopy (POM). It was found that wood fibres with different size had different effects on the melting temperature, crystallinity and crystal growth rate of PLA in WF/PLA composites during isothermal crystallization process. The melting peak of PLA component shifted to higher temperature by increasing fibre size. The middle size WF (diameter=0.012-0.050 mm, length =0.3-4.0 mm) could promote heterogeneous nucleation of PLA, increased crystal growth rate and crystallinity of PLA in composites. But the large size WF (diameter=0.07-0.50 mm, length=2-10 mm) would hinder crystal growth, then reduce crystallinity. On non-isothermal crystallization phase the results show that proper WF size could increase the overall crystallinity, melting peak temperature and crystallization temperature. But the large size WF (diameter=0.07-0.50 mm, length=2-10 mm) is too big to promote crystal growth. POM shows WF as a sort of heterogeneous nucleating agent could promote nucleus forming at the WF surface and confirmed the large WF would hinder crystal growth.