In this work, the preparation and characterization of new cement mortars reinforced with nanofibrillated cellulose fibres has been carried out. The effect of the fibre fibrillation degree on the mechanical performance of the obtained composites is presented and analyzed, taking into account the reinforcement final size depending on the intensity of the mechanical treatment applied. On the one hand, conventional pulps have been obtained by subjecting sisal fibres to a soft mechanical treatment and, on the other hand, nanofibrillated cellulose pulps have been prepared by the application of a high intensity refining process. These pulps were incorporated and homogenously dispersed in cement mortars, and subsequently, the resulting pastes were cured for 28 days. The mechanical performance of the different composites was determined by flexure tests