  Let $\phi(z)=(\phi_1(z),...,\phi_n(z))$ be a holomorphic self-map of $B_n$
and $\psi(z)$ a holomorphic function on $B_n$, and $H(B_n)$ the class of all
holomorphic functions on $B_n$, where $B_n$ is the unit ball of $C^n$, the
weight composition operator $W_{\psi,\phi}$ is defined by $W_{\psi,\phi}=\psi
f(\phi)$ for $f\in H(B_n)$. In this paper we estimate the essential norm for
the weighted composition operator $W_{\psi,\phi}$ acting from the Hardy space
$H^p$ to $H^q$ ($0<p,q\leq \infty$). When $p=\infty$ and $q=2$, we give an
exact formula for the essential norm. As their applications, we also obtain
some sufficient and necessary conditions for the bounded weighted composition
operator to be compact from $H^p$ to $H^q$.
