•Estimation method of the fundamental frequency in the EOL fuel assembly is suggested.•Bending energy is used to identify the fundamental frequency.•Prediction error can be minimized when the deflection shape resembles the first mode shape.•Test examples show that the suggested method is reliable. This paper develops a methodology to predict the End-Of-Life (EOL) fundamental frequency of a Pressurized Water Reactor (PWR) fuel assembly using lateral bending test data. Bending energy, developed from the lateral bending tests, is utilized to predict the first mode natural frequency. It is shown that the estimation provides reasonable results when the deflected shape is close to the first mode shape. One can evaluate the structural integrity against earthquakes by utilizing a simulation tool that uses the physical information of the fuel assembly. One of the important pieces of information for the simulation is the first natural frequency of the fuel assembly, because the dominant frequency components of the reactor motion are close to the fundamental frequency of the fuel assembly. When the EOL information is unavailable, the unknown EOL fundamental frequency can be identified directly through tests. However, it is almost impossible to build a specific fuel assembly when the fuel assembly is no longer produced. Therefore, this paper suggests using the load-to-deflection curve from the static bending test to predict the fundamental frequency of the EOL fuel assembly. Estimation errors are also discussed, and it is shown that the errors can be minimized using the bending energy considering the deflected bending shape that is the most similar to the fundamental mode shape. Two test examples are discussed, and the results show that the suggested method provides reliable results.