  The orthogonal decomposition of the Webster curvature provides us a way to
characterize some canonical metrics on a pseudo-Hermitian manifold. We derive
some subelliptic differential inequalities from the Weitzenb\"ock formulas for
the traceless pseudo-Hermitian Ricci tensor and the Chern-Moser tensor of
Sasakian manifolds with constant pseudo-Hermitian scalar curvature and Sasakian
pseudo-Einstein manifolds respectively. By means of either subelliptic
estimates or maximum principle, some rigidity theorems are established to
characterize Sasakian pseudo-Einstein manifolds among Sasakian manifolds with
constant pseudo-Hermitian scalar curvature and Sasakian space forms among
Sasakian pseudo-Einstein manifolds respectively.
