This paper proposes a formal verification strategy of grid frequency control using reachability analysis. Reachability analysis calculates reachable sets, which are all possible evolution of system states, or output variables, given a bounded input uncertainty. Contrary to classical grid frequency control schemes that are generally tuned based on multiple simulations, reachability analysis provides a formal guarantee for the performance of the controller in one computation stage. The proposed method is applied on the IEEE 9-Bus system. The accuracy of reachable sets is validated by simulation results with randomized inputs. In addition, the paper analyses the effect of control parameters and input uncertainties on the reachable sets. Thus, the operator can verify if their tuned frequency controller could violate any mandated grid directives without performing large number of simulations