External bonding of ultra-high performance fiber reinforced concrete (UHPFRC) plates to reinforced concrete (RC) beams has been found to be an effective technique for shear strengthening. In order to obtain a clear understanding of shear strength of RC beams strengthened with UHPFRC, a numerical finite element model has been developed using the finite element package Abaqus/standard. The proposed model was verified and used to investigate crucial parameters of the examined technique. Parameters, which are mainly related to the design of this technique, such as effect of shear span to depth ratio, effect of UHPFRC plates' thickness, effect of the UHPFRC plates' reinforcement ratio, effect of the individual contribution of longitudinal side strengthening, and effect of the UHPFRC compressive strength on mechanical performance of RC beams strengthened in shear with prefabricated UHPFRC plates have been analyzed in the proposed research. The investigation resulted in a number of important conclusions reflecting the effects of the studied parameters on the behavior of the strengthened beams.