Flow circulation in natural circulation boilers is analysed. Based on a simplified model with a single tube loop employing homogeneous equilibrium flow assumption, a simple and universal relationship is obtained between non-dimensional circulation velocity and heating load newly introduced. The analysis clearly shows the importance of the hydraulic resistance of the downcomer. The effects of slip between steam and water are also examined and compensated by introduction of a correction multiplier to the circulation velocity. Then, a practical method to estimate flow circulation of boilers composed of a large number of evaporating tubes and several downcomers is developed. Finally, the Ishigai-Akagawa's flow criterion for evaporating tube is modified according to the author's results, and by using this new flow criterion and the downcomer resistance criterion the design practice of recent commercial boilers is surveyed.