Rational harmonic injection locking (RHIL) can make the whole circuit simpler and more flexible than harmonic injection locking (HIL) does. A common formula of the synchronizing band △(s/m)max of the RHIL is derived and aualysed in this paper. Itshows that the formula of the synchronizing band of HIL given by I. Schmideg (1971) is a special case.A method which is used to expand the RHIL s synchronizing band with full-pass network, is presented. With this method, a 2/3 injection-locking VHF divider made of IC FZIC is developed. Its stability factor n=f(3/2)max)/f[-40+85℃]4. For the original circuit, n0.6. It makes the RHIL of better practical value.