  We present a possible explanation to the tiny positive cosmological constant
under the frame of AdS$_5$ spacetime embedded by a dS$_4$ brane. We calculate
the dark energy density by summing the zero point energy of massive scalar
fields in AdS$_5$ spacetime. Under the assumption that the radius of AdS$_5$
spacetime is of the same magnitude as the radius of observable universe, the
dark energy density in dS$_4$ brane is obtained, which is smaller than the
observational value. The reasons are also discussed.
