This paper introduces structural assessment and strengthening of a rein forced concrete pylon which experienced torsional deformation. The pylon with a hexagonal hollow section is the lower part of the cable-stayed bridge under construction. As a floating crane hit the cantilever end of the super-structure of the bridge constructed through cantilever erection method, torsional deformation of 1.2 degree occurred along the pylon. After the collision, torsional cracks were observed on the outer and inner surfaces of the hollow pylon. The structural assessment of the pylon was conducted with nonlinear finite element analysis to simulate structural behavior. The result showed that yields of transverse and longitudinal reinforcements were not significant when the floating crane collided. After the super-structure of the bridge was restored to the original position, there were residual strains in the reinf orcements, but stresses almost returned to near the origin. Therefore, it could be concluded that there were no significant damage to the pylon. In spite of little deterioration on the structural capacity, the client decided to strengthen the pylon by wrapping it with GFRP sheets after repairing cracks.