Natural circulation is considered one of the most significant thermal–hydraulic processes that causes the fluid to flow along a closed loop without the need for an external driving force and is used in the majority of the new and creative nuclear reactor designs. The phenomenon of natural circulation has been the subject of numerous studies in recent years. This study aims to give readers a basic understanding of the thermal hydraulic aspects of natural circulation throughout experimental observations. By examining how a natural circulation loop behaves under various power conditions. The experiment was conducted at Harbin Engineering University's reduced scale natural circulation loop, which was used for a number of studies aimed at examining various aspects of natural circulation. The main loop consists of an electric heated test section, a riser, a condenser, a down-comer, measuring instrument and relevant pipes. Condensation loop provide cooling water to secondary side of condenser and provide heat sink for natural circulation Loop. The test section is uniformly heated by a DC power, and the loop has a shell-and-tube exchanger for hot water cooling and the working medium of the loop is deionized water. The measurement for the experiments was obtained with 0.2 and 0.3 MPa system pressure and Different working conditions at same pressure reached by increasing gradually the heating power starting from 5 to 14 KW. The analysis of the experiment results illustrated the variation of the mass flow rate, the inlet and outlet temperature, and the wall temperature along the heated section with heating power. All these parameters increase with heating power and show different oscillations in high power. The results of the experiment demonstrated that flow instability was the source of the temperature oscillation at high power. Also, the observation of the flow reversal has been highlighted.