Analysis of The results for one of the most important experimental tests on masonry arches strengthened at their extrados or at their intrados by fiber reinforced polymer (FRP)strips;shows that the presence of the fibers prevents the brittle collapse that occur in plain arch because of formation of four hinges, therefore (depending on position and amount of reinforcement) in strengthened arches, there are three possible mechanisms : 1- masonry crushing , 2- detachment of fibers, and 3- sliding along a mortar joint due to the shear stresses. Some analytical approaches describing these mechanisms are discussed and a comparison between the theoretical values that give and the experimental results was performed to show the agreements , according to analytical study of the experimental results strengthening by fibers enhances the strength and ductility of strengthened arches, the width of strips and the bond between them and masonry are so important to perform optimum strengthening.