Acad Sinica, Inst High Energy Phys, Beijing 100039, Peoples R China
We calculate the vacuum energy density by taking account of different massive scalar fields in AdS spacetime. It is found that the mass spectrum of a scalar field in AdS spacetime is discrete because of a natural boundary condition. The results match well with the observed cosmological constant. (C) 2002 Elsevier Science B.V. All rights reserved.