The thermal hydraulics of supercritical water under forced-, mixed convection and natural circulation conditions is not fully understood. In order to study the thermal hydraulic behaviour of this fluid under natural circulation conditions a small size, closed experimental loop has been designed and built. The thermal hydraulic phenomenon occurring in the loop can be measured by thermocouples mounted onto the outer surface of the heated tube wall, absolute and differential pressure transducers and a flow meter; moreover, simultaneously can be visualized by neutron radiography techniques. This paper describes the loop itself, the process of the experiment with the measurement techniques, the data acquisition system applied and the results got during the first measurement series. Based on the results of the first measurement series, it was found that the measured part of the steady state characteristic is independent from the system pressure. A slight dependence of steady state characteristic on the inlet temperature can be identified: the higher the inlet temperature the higher the mass flow rate. The total pressure drop and its components seem to be independent from the system pressure but strongly dependent on the inlet temperature due to the influence of bulk-fluid temperature on the relevant thermophysical properties (density and dynamic viscosity). The pressure drop due to acceleration of flow found to be negligible next to the two dominant components, the pressure drop due to frictional resistance and due to gravity, The coupled evaluation of the radiographic images and the thermophysical properties of wafer have shown that the main driving force behind the decrease of the neutron attenuation is the decreasing water density as the bulk-fluid temperature increases. The reverse of this relationship could be exploited during the validation of future Monte Carlo simulations. (C) 2016 Elsevier Ltd. All rights reserved.