Single phase natural circulation is a heat transfer mechanism of great interest in various energy systems, including solar heaters, nuclear reactors, geothermal power production, engine and computer cooling. The present paper deals with an experimental study on the influence of power steps on the behavior of a single-phase natural circulation loop. In particular two sets of experiments were performed: constant power and variable power. Several parameters such as the amplitude of power steps as well as the period of the oscillations were experimentally investigated. The runs at constant power were carried out at different power levels from 500 W to 3000 W. The experiments at variable power were carried out for the values of 500 W, 1000 W, and 2000 W. The amplitude of steps was +/-50% or +/-20% of the input power, whereas the period of the oscillations varied between 50 s and 7200 s. All tests, both at constant power and variable power, show always an unstable behavior, i.e., temperature oscillations across the heated sections occur in the fluid. Moreover, the amplitude and the frequency of the oscillations increase as the input power increases. The thermal inertia of the loop plays a role in case of smaller time steps. (C) 2016 Elsevier Ltd. All rights reserved.