The real-time center of inertia frequency plays an important role in power system stability analysis and control. This letter proposes a robust approach to identify power system center of inertia frequency with consideration of system uncertainties and synchrophasor measurement quality. A model decoupling strategy is first presented by taking measured generator active power as the swing equation input, whereas remaining the bus frequency as output. This allows deriving the linear discrete-time state-space form, which will be used by the robust Kalman filter for generator rotor speed, angle, and inertia estimation. The robust Kalman filter is mandatory as the measured frequency can change abruptly due to impulsive noise or system sudden changes. Simulations carried out on the IEEE 39-bus system validate the effectiveness and robustness of the proposed approach in the presence of various sources of uncertainties.