  The energy band structure and energy splitting due to quantum tunneling in
two weakly linked Bose-Einstein condensates were calculated by using the
instanton method. The intrinsic coherent properties of Bose Josephson junction
were investigated in terms of energy splitting. For $E_{C}/E_{J}\ll 1$, the
energy splitting is small and the system is globally phase coherent. In the
opposite limit, $E_{C}/E_{J}\gg 1$, the energy splitting is large and the
system becomes a phase dissipation. Our reslults suggest that one should
investigate the coherence phenomna of BJJ in proper condition such as
$E_{C}/E_{J}\sim 1$.
