A theoretical model is developed for the effect of the operating parameters and the geometric parameters on the Kaplan turbine performance under design and off-design conditions. In the theoretical model, the incidence loss at the runner inlet that exists under off-design conditions for both positive and negative incidence angles is taken into account. The runner internal loss and the draft tube loss, which exist even under design conditions, are also taken into consideration through the use of simple and reasonable empirical expressions. The experimental works were carried out to validate the theoretical results. An optimization technique is adopted to obtain the operating parameters that achieve the best possible efficiency of a Kaplan turbine. A chart is deduced for detecting the suitable values of the operating parameters that achieve efficiency higher than 88% for different geometric parameters of the axial flow turbine, constant runner inlet blade angle. Comparisons of the obtained results with the present experimental and available published works show an acceptable agreement