Dark Energy Density in Brane World

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.