One of the greatest impact dispersion is caused by zero roll-rate for spinning missiles during their re-entry flights. The mechanism of roll-rate through zero is analysed. The altitude of zero roll-rate for including roll damping influence and the impact dispersion as a result of zero roll-rate are formulaed. Finally，we compare six degrees of freedom simulation results of ballistic re-entry missiles motion with the computing results of approximate formula. The comparison shows the feasibility of the approximate analysis.