Abstract: Based on experimental results of four reinforced concrete box beams strengthened with fiber reinforced polymer (FRP), a 3-D finite element model was established and the nonlinear finite element method was used to study the torsional behaviors of FRP strengthened reinforced concrete box beams under combined loadings of bending moment, shear and torque. The numerical results of torque-twist angle relationships, strains in steel bars and FRP, forces of interfacial element showed good agreement with the experimental results. This indicates that the nonlinear finite element method can simulate the torsional behavior of FRP strengthened box beams. Further investigation was conducted with respect to the shear-torque behavior of FRP strengthened reinforced concrete box beam. The calculated results of seven numerical box beams indicated that dimensionless correlation of shear and torque strength of FRP strengthened RC box beams under combined bending, shear and torque was linear.