Critical heat dux has been measured during natural circulation boiling of water on uniformly heated outer tube in vertical annular tube. The experiment was carried out using water at atmospheric pressure for the clearance of 1.0-4.0 mm, the heated tube diameter of 9-17 mm, and the annular tube length of 100-1000 mm. The similarity of CHF between annular configurations of either inner or outer heated tubes and a simple heated tube can be clearly elucidated based on the characteristics of heated equivalent diameter. The CHF measured for s = 1 mm can be predicted well by existing correlation for the annular tube and for s = 3 and 4 mm by existing correlation for the single tube.