  Three most long and dense VLBI nutation series obtained at the Goddard Space
Flight Center, Institute of Applied Astronomy, and U.S. Naval Observatory were
used for investigation of the Free Core Nutation (FCN) contribution to the
celestial pole offset. Some recent studies have showed that the FCN period
or/and phase does not remain constant, but varies in a rather wide range of
about 410--490 days (for equivalent period). To implement this result in the
practice, a new FCN model with variable amplitude and period (phase) is
developed. Comparison of this model with observations shows better agreement
than existing one. After correction of the differences between observed VLBI
nutation series and the IAU2000A model, they decreased to a level about 100
microarcseconds.
