  Based on the principle of relativity and the postulate of invariant speed and
length, we propose the theory of special relativity with cosmological constant
${\cal SR}_{c,R}$ if the invariant length whose square is the inverse of the
one-third cosmological constant of the universe. It is on the Beltrami-de
Sitter spacetime ${\cal B}_R$ with de Sitter invariance. We define the
observables of free particles and generalize famous Einstein's formula. We also
define two kinds of simultaneity. The first is for local experiments and
inertial motions. The second is for cosmological observations. Thus there is a
relation between the relativity principle and the cosmological principle. We
predict that the 3-d cosmic space is then of positive spatial curvature of
order cosmological constant. The relation between ${\cal SR}_{c,R}$ and the
doubly special relativity is briefly disucssed.
