This study proposes an inverse method to estimate input forces of nonlinear dynamic systems. The present estimation method is composed of the Extended Kalman Filter (EKF) and a Recursive Least Squares Estimator (RLSE). By using the inverse method, input forces acting on nonlinear structural systems can be estimated from measured dynamic responses. In this work, laboratory tests of input forces estimation of a cantilever beam with a lumped-mass on the free end are performed to verify the practicality and accuracy of the proposed algorithm. In the experimental work, the beam was excited by harmonic forces. The estimation results demonstrate that the application of the input force estimation method to nonlinear dynamic systems is successful.