  In this paper we use a unified way studying the decay estimate for a class of
dispersive semigroup given by $e^{it\phi(\sqrt{-\Delta})}$, where $\phi:
\mathbb{R}^+\to \mathbb{R}$ is smooth away from the origin. Especially, the
decay estimates for the solutions of the Klein-Gordon equation and the beam
equation are simplified and slightly improved.
