In order to investigate a criterion on failure mode of the RC beams flexural strengthened with FRP sheet under impact loading, falling-weight impact tests for RC beams strengthened with the AFRP sheet were carried out taking FRP sheet volume as variable. The results obtained from this study were as follows: (1) applying AFRP sheet bonding method, impact resistant capacity of the beams can be upgraded; however, (2) in the case of compression failure mode statically, the beams reach ultimate state with sheet rupturing; (3) in the case of sheet debonding failure mode statically, the beams reach ultimate state with sheet debonding.