; Lu Xiao-Fu] Sichuan Univ, Sch Phys, Chengdu 610065, Peoples R China
; [Lu Xiao-Fu] Chinese Acad Sci, Inst Theoret Phys, Beijing 100190, Peoples R China
; [Lu Xiao-Fu] CCAST World Lab, Beijing 100080, Peoples R China
On the basis of assuming that the narrow state X(3872) is a molecule state consisting of D-0 and (D) over bar (*0), we apply the Mandelstam generalization of the Gell-Mann-Low method to calculate the matrix element of quark current between the heavy meson states described by Bether-Salpeter wave function. In calculation of the matrix element of quark current the operator product expansion is used in order to include the nonperturbative contribution of the vacuum condensates. In this scheme we calculate the mass of X(3872). We believe that this scheme is closer to QCD than the previous work.