; Zhang, C. M.
; Zhao, Y. H.] Chinese Acad Sci, Natl Astron Observ, Beijing 100012, Peoples R China
; [Lin, Y. F.] Tsinghua Univ, Dept Phys, Beijing 100084, Peoples R China
; [Lin, Y. F.] Tsinghua Univ, Tsinghua Ctr Astrophys, Beijing 100084, Peoples R China
; [Yin, H. X.] Shandong Univ, Sch Space Sci & Phys, Weihai 264209, Peoples R China
; [Song, L. M.] Chinese Acad Sci, Inst High Energy Phys, Beijing 100049, Peoples R China
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-disc boundary, which corresponds to the coherence of upper kHz QPO. The theoretical model for the Q-factor of upper kHz QPO is applied to the observational data of five Atoll and five Z sources, and the consistence is implied.