A new database of natural circulation in a vertical annulus is under development in the Multiphase Thermofluid Dynamics Laboratory (MTDL) at the University of Illinois at Urbana-Champaign. The focus of the dataset is on the two-phase phenomena of boiling, condensing, flashing, and saturated flow in a vertical channel under natural circulation. The 5.03 m annular test section with inner and outer diameter of 19.1 mm and 38.1 mm respectively houses a 3 m internally heated annular section followed by a 2.03 m unheated annular section. Five measurement ports, two in the heat section, one at the heated to unheated boundary, and two in the unheated section provide measurement of pressure and local measurements of void fraction, interfacial area concentration, gas velocity, Sauter mean diameter, and liquid temperature. Additional pressure and liquid temperature measurements are provided at important locations around the natural circulation loop. The dataset is designed for validation of one-dimensional system codes under natural circulation for pressures up to 1 MPa, which are important to passive cooling approaches to reactor safety.