Hydraulic model tests of the spillway of Yukar Kalekoy Dam and Hydroelectric Power Plant (HPP) were carried out at Hydromechanics Laboratory of Middle East Technical University (METU), Ankara. In order to investigate the flow conditions in the approach channel, over the weir structure, around the spillway piers, along the spillway chute channels and after the flip buckets, and also to observe the performance of the aeration devices, a model of 1/70 length scale was constructed. The model area covers an about 400 m x 450 m section of the Yukar Kalekoy reservoir, the whole spillway structure and a part of the river area of about 150 m x 200 m downstream of the flip buckets. Experiments were carried out at various discharges; QPMF, Q(1000) and Q(100) with fully open radial gates as well as with partly open radial gates at Q(1000). Flow depths, pressure heads of the flows along the spillway structure were measured, cavitation indexes and air concentrations of the flows were determined. From the observations and experimental results, it was concluded that the flow conditions in the approach channel of the spillway, over the weir structure, around the spillway piers, along and after the flip buckets are quite good from the hydraulics point of view. However, the flows were not sufficiently aerated in the spillway chutes, in Channels I and II, after the available aerators presented in the original design of the spillway. After the experiments conducted on the "original model" it was decided that some modifications should be done. In order to improve the performance of the available aerators, the heights of the aerator deflectors were increased by 0.20 m by keeping their lengths in the flow direction almost as before. (C) 2016 The Authors. Published by Elsevier Ltd.