Exceptional hydraulic characteristics of these spillways, resulted in extensive use of vertical shaft spillways in stone-bed-rivers as well as narrow valleys with steep slopes. In such conditions other types of spillways are almost inappropriate or are not economical. These spillways are hydraulically imposed with several problems, owing to formation of surface vortices at their intakes. Generation of air-core vortices at the spillway intake, leads to cavitation and reduction of the spillway flow discharge. Vortices are commonly appear until a critical submergence depth is attained. To overcome these destructive effects and to diminish the air entrainment into the shaft, a number of devices and solutions have been proposed. Among these the most applicable one is establishment of anti-vortex plates at the spillway crest. These plates delay the generation of air-core vortex flow and directs the streamlines towards the shaft’s center radially. Recently, an innovative shape of inlet namely circular Piano-Key (CPK) inlet has been proposed to diminish the vortices at the intakes, thereby to increase the discharge capacity of shaft spillways. Having a simple configuration, CPKs are economic hydraulic structures, being able to suspend the vortex flow at the spillway entrance. Compared to a simple circular shaft spillway inlet, the effective length of a CPK inlet is increased, resulting in the increased flow discharge of up to 4 times the flow discharge of the corresponding simple shaft spillway. The present study investigates the hydraulic characteristics of flow at vertical shaft spillways equipped with CPK inlets, based on model experimentation. Experiments were performed in a cylindrical galvanized reservoir of 2 m diameter and 1 m high, in which a vertical shaft is placed at center of the reservoir bottom. CPK inlets with different length, height and key angles were assembled over the entrance of the vertical shaft and different hydraulic aspects of flow were studied. Water flow head over the inlets were measured by means of a point gauge of 1 mm precision, as well as by a set of piezometers installed under the reservoir bottom. Instantaneous velocity measurements were made applying an Acoustic Doppler Velocimeter (ADV). CPK inlets act as anti-vortex devices, reducing the tangential flow velocity component. Accordingly the water head decreases for a given flow discharge compared to the simple vertical shafts and Morning glory spillways.The measured data were analyzed using a nonlinear regression analysis and a statistical software namely , resulting in different correlations for determining the discharge coefficient, orifice flow threshold submergence and critical submergence depth. To validate the obtained correlations several criteria including the NRMSE, WQD and R 2 for each derived equation were examined. Finally the best correlations with the maximum coefficient of determination and the minimum error values presented herein. Key words : Critical submergence depth, discharge coefficient, piano key weir, surface vortex, vertical shaft spillway.