The grid adjustment is seriously affected by the instability of Pumped Hydro Energy Storage plants (PHES), one of which is the phenomenon of Positive Slope (PS) in pump turbine. In this paper, two positive slopes were experimentally captured on one curve of the discharge-head performance in a pump turbine while only one positive slope is observed on one pump performance curve in general pumps. The similarities and differences of two positive slopes are demonstrated using data obtained with this pump turbine model. It is found that two positive slopes are caused by two different stalls, respectively. For the first positive slope, the stall with five stall cells in the vane-less domain (between impeller and wicket gates) is induced to play a key role. On each pump performance curve, with the smaller opening of guide vanes, the discharge of the first PS is less, until the first PS and the second PS becomes one PS. For the second positive slope, significant complex vortexes in the impeller inlet, the variations of cavitations and a lower signal of pressure fluctuation are observed. The discharges of the second positive slopes almost keep stable with the different wicket gates' openings. For their common characteristics, hysteresis phenomena are observed in both PS and are influenced by cavitations. (C) 2020 Published by Elsevier Ltd.