  {We investigate the coherence of the twin kilohertz quasi-periodic
oscillations (kHz QPOs) in the low-mass X-ray binary (LMXB) theoretically. The
profile of upper kHz QPO, interpreted as Keplerian frequency, is ascribed to
the radial extent of the kHz QPO emission region, associated with the
transitional layer at the magnetosphere-disk boundary, which corresponds to the
coherence of upper kHz QPO. The theoretical model for Q-factor of upper kHz QPO
is applied to the observational data of five Atoll and five Z sources, and the
consistence is implied.
