The use ofCement-based composites natural fibers in cementitious compositesComposites still presents a limitation to their durability in the alkalineAlkaline matrix. Some treatments on the fiber are carried out in order to reduce its degradation in the matrix, such as surface treatment with NaOH. The objective of this study is to evaluate the durability of guaruman fiberGuaruman fiber (Ischinosiphon koern) with surface treatment with NaOH as a possible reinforcement for coating mortarsCoating mortar. For this, compositesComposites were manufactured with and without treatment in proportions of 0% (reference), 0.5%, 1% and 1.5% in relation to the mass of Portland cement with a ratio of 1:3:0.66 (cement:sand:water/cement factor). The experimental program included testing with durabilidade wetting and drying. Based on the results obtained, it was found that matrices containing 0.5% and 1% additions of fibers were those that showed the best performance, suggesting that guaruman is a promising new material for reinforcement in coating mortarsCoating mortar.