Flow reversal phenomena in natural-circulation boilers are studied analytically according to both the homogeneous-flow model and the slip-flow model. In the latter model Thom's pressure drop correlation is used. The pressure drop between the upper steam-water drum and the lower header is assumed not to be affected by inception of reverse flow in an evaporator tube which will be analyzed later.The mechanism of flow reversal is examined through analysis of the static flow characteristics of the evaporator tube in the non-dimensional form. Some important dimensionless parameters are introduced and their effects on inception of reverse flow are studied.It was found that the criterion of flow reversal is correlated practically with only three dimensionless parameters. Finally, an approximate method to predict flow reversal is put forward.