Cement mortar is a broadly used construction material. Adding fibres into the fresh mortar is an effective reinforcing technique to overcome the drawbacks related with the little tensile and flexural strength. Using natural fibres (mostly vegetal) is environmental sustainable and competitive. In particular, alfa fibre, which is produced in Mediterranean area, is herein considered. The main objective of this paper is analysing the mechanical performance of strengthening mortars using alfa fibre. Flexural and compressive tests have been performed and the experimental results showed a significant increase of the flexural strength and the toughness factor, whereas the compressive strength was slightly reduced when adding alfa fibres. The methodology used for measuring the toughness factor has been successfully adapted for this research and, thus, it might be applied to other materials. Different fibre dosages and lengths have been tested and the influence of these parameters have been analysed concluding that adding short fibres is the best option because of the better workability and the greater flexural performance.