Aging and deterioration of existing steel structures necessitates the development of simple and efficient rehabilitation techniques. Strengthening with bonded Carbon Fiber Reinforced Plastic plate has been rarely applied to steel structures, although it has often been applied to concrete structures. To enhance the usage life of existing steel buildings, a strengthening method with bonding Continuous fiber reinforced plastic (FRP) plates has been developed by authors. An analytical model is required that can predict the flexural rigidity and yield moment as well as the stresses induced in FRP plates of rehabilitated members. This paper reports the results of bending tests conducted on a Narrow H -shaped steel beam before and after rehabilitation using FRP plates. These results show enhanced flexural rigidity and the moment carrying capacity of the beam due to bonding FRP plate. An analytical model is also proposed that can predict the flexural rigidity of rehabilitated steel beam.