  We consider the effect of an external periodic force on chimera state in the
phase oscillator model proposed by Abrams et al. [Phys. Rev. Lett, v. 101,
00319007 (2008)]. Using the Ott--Antonsen reduction the dynamical equations for
the global order parameter characterizing the degree of synchronization are
constructed. The regions of the global order parameter frequency locking by an
external periodic force are constructed. The possibility of stable chimeras
synchronization and unstable chimeras stabilization are established.
