A reinforcement structure for a bicycle includes a rim, and a plurality of carbon-fiber locking blocks, spokes and screw members. The carbon-fiber locking blocks are locked on the rim by the screw members and the spokes. Each of the carbon-fiber locking blocks has a first side formed with a planar surface rested on a respective screw member and a second side formed with an arc-shaped surface closely rested on the rim. Thus, the external force applied on each of the screw members is evenly distributed by the arc-shaped surface of the respective carbon-fiber locking block, thereby preventing the screw members from being distorted due to a stress concentration, so as to enhance the safety of riding the bicycle.