A geometrically similar model of the proposed Advanced Heavy Water Reactor being designed and built by Bhabha Atomic Research Centre has been conceived and built at Indian Institute of Technology, Bombay. The basic objective of setting up such a facility and conducting experimental studies was to simulate the thermal-hydraulic behavior during the startup and also during part or full power operation. Owing to hydrodynamic instabilities observed in natural circulation system, it is necessary to know the stability boundaries. In this paper, the details of the experimental facility, experimental studies, and numerical simulations carried out using RELAP5/MOD 3.2 are discussed. Type I and type II stability boundaries and stable two-phase operation zones have been experimentally obtained at various pressures and inlet subcooling and the results are correlated using appropriate nondimensional numbers. Further, the experimental facility is modeled using the thermal-hydraulic code RELAP5/MOD 3.2. The single-phase pressure drops measured in the loop are used to estimate the appropriate loss coefficients used in the model. The model is then used to predict stability boundaries, and the predictions are compared with the experimental findings. The details of the experimental model and simulation results are presented. The capability of RELAP5/MOD 3.2 to predict the flow oscillations during instabilities is discussed.