Abstract The natural frequencies of a turbine can be calculated from numerical methods. By comparing these natural frequencies with excitation sources, one can know the danger of a resonance and a possible failure in a component of the turbine. Therefore, it is often very important to have an accurate numerical model of the turbine to determine these natural frequencies. There are not many publications on the determination of the natural frequencies of reduced-scale models of Kaplan turbines. More papers exist for pump turbines or Francis turbines. For real Kaplan turbines, very few experiments can be found to determine mode shapes and natural frequencies. In this paper a Kaplan turbine of 37MW (maximum power), 12.5m (maximum head) and 50 m3/s (maximum flowrate) was tested. The turbine was equipped to determine the natural frequencies of the runner in air. For this purpose, one accelerometer in each blade of the runner was installed and a total of 16 impacts were done in each blade. Frequencies and mode shapes were obtained. In parallel, a numerical model was obtained. Numerical and experimental results were compared and an accurate numerical model is presented. With this numerical model the natural frequencies of the runner in water were calculated.