Fundamental two-phase flow behavior under natural circulation conditions is studied both experimentally and analytically. Natural circulation characteristics for both drum-type and chimney-type reactors are experimentally investigated focusing on the flow patterns, flow rate and two-phase behavior inside the vessel. Analytical studies are performed to further understand the natural circulation characteristics of an actual plant and the mechanisms of geysering-like instabilities. Experimental and analytical results confirm that sufficient and stable flow is achievable in a natural circulation BWR.