Abstarct: High-Performance Fiber-Reinforced Cementitious Composites (HPFRCCs) are a new type of fiber reinforced composite that has emerged in recent decades. The most important aspect of HPFRCCs that sets them apart from ordinary fiber reinforced concretes is their strain hardening. As a result, HPFRCCs are more ductile than conventional concrete, making them ideal for some structural applications. The main goal of this thesis is to investigate the effect of existing substrate strain on the behavior of RC beams strengthened by HPFRCC. A code in MATLAB program was created for this purpose. Under this code, the moment-curvature curve of the beam is first produced using the stress-strain curve of the constitutive material in tension and compression. Then, the load-deflection curve of the beam under any pattern of the load is derived utilizing the existing relationships of materials. Three experimental test results were used to validate the code, and the results of the analytical model show good agreement with the test results. The model is capable of predecting different stages of beam behavior. Various parameters, such as thickness and the type of reinforcing laxyer, were investigated in this research. By applying varied preloads before attaching the laxyer on the control beam, the goal of this thesis can be achieved. The results indicated that the substrate strain has no considerable influence on the ultimate strength and ductility of the beam retrofitted by composites with strain hardening behavior. In composites with strain softening behavior, however, the strain corresponding to the maximum tensile stress (ε_pl) is critical in assessing the influence of substrate strain. As a result, in composites with greater ε_pl, the effect of substrate strain on the beam strength is about 1.6 percent, while in composites with less ε_pl, this value is near to 12 percent. The same is true for ductility; the effect of the substrate strain in composites with greater ε_pl, is less than 4.3 percent, but it may reach 61 percent in the other one.