The near-surface mounted (NSM), in which strips or rods are fixed by an adhesive in slits formed inside the concrete cover, and the externally bonding (EB), in which the reinforcement is glued to the concrete surface, are well-known strengthening methods. In the present work, an experimental and numerical study was carried out on reinforced concrete (RC) beams strengthened with NSM and EB-reinforced high-strength concrete (RHSC) layers. The RHSC layers were reinforced by steel or glass fiber reinforced polymer (GFRP) rods and bonded to the RC beams without and with a concrete cover. Seven specimens were cast and loaded in flexure up to failure to investigate the effect of RHSC layer position and reinforcement on the RC beams’ efficiency. The threedimensional finite element program (ABAQUS) verified the experimental results numerically. Also, a parametric study was numerically performed to examine the factors affecting the flexural performance of the strengthened RC beams using NSM and EB RHSC layers. The strengthened beams by NSM steel RHSC layers increased the load capacity by 147.9% compared to the control beam (CB). Changing the RHSC layer reinforcement from steel to GFRP bars enhanced the load’s efficiencies from 147.9% to 172.0% compared to the CB. Decreasing the width and reinforcing area of the RHSC layer decreased the beam load capacity by about 17.5 %. Due to their longer bending arms, the EB RHSC layers showed lower load capacity than the NSM ones, which accelerated the EB RHSC failure. When the carbon fiber-reinforced polymer bars were utilized to reinforce the HSC layers, the strengthened beams showed higher load efficiency than those strengthened with HSC layers reinforced by GFRP rods.