In this research, alkaline pre-treated Guadua Angustifolia fibres were refined in a PFI mill refiner at different speeds (1500, 4500, 7000 and 10000 rev) and used as reinforcement in cement composite boards CCB.To determine physical changes after refining, Water Retention Value (WRV), Fibre length and SEM images were evaluated.Additionally, crystalline indices were calculated by Segal method from XRD spectra.With the aim to evaluate the effect of degree of refining in the mechanical response of CCB its flexural strength was evaluated after 7 and 28 days of curing.Results show an increasing of WRV up to 4500 rev of refining, followed by a decrease once this point of refining is overcome.Likewise, flexural strength of composites shows a similar behaviour as WRV, exhibiting an optimal value between 4500 and 7000 rev of refining