Abstract: For natural circulation, it is shown that parallel flow in the tubes of vertically inverted U-tube team generator (UTSG) can be non-uniform, and reverse flow occurs within some tubes. A flow model of one-dimension, steady state was formed. According to the linear perturbation theory, the mechanism of reverse flow was studied. Reverse flow is considered as a typical Ledinegg instability. It is nonlinear relationship between U-tube length and characteristic pressure drop when instability happens. And there exists characteristic U-tube length which has the largest characteristic pressure drop.