Due to the complexity and difficulty involved, the interfacial bond-slip relationship of externally bonded (EB) fiber-reinforced polymer (FRP)-to-concrete joints has been investigated, mainly through experimental testing. This work investigates the interfacial behavior using the finite-element method (FEM). Damage plasticity theory is employed to model concrete material and the pull-off test of EB-FRP-to-concrete joints is simulated by FEM to study the interfacial behavior. Debonding process, failure mechanism, local bond strength distribution, and characteristics of the local bond-slip relationship are examined. It is found that the local bond-slip relationship has a shape different from all existing bond-slip models. However, such change in shape does not make a significant difference to the global interfacial load-slip response.