For flow instabilities in inverted U-tubes steam generator (SG),a dimensionless analysis is made for flow equation.Criteria of flow stabilities (characteristic Reynolds number) are derived according to small perturbation analysis.With calculation for a certain type of SG,effect of inlet fluid temperature and U-tube bend radius on the criteria are studied.It shows that the characteristic Reynolds number increases with the increase of inlet fluid temperature.Relations between U-tube bend radius under different inlet temperatures are complex.It is useful for calculation of flows as well as space distribution of reverse flow in a SG.