  We discuss the minimum of Willmore functional of torus in a Riemannian
manifold $N$, especially for the case that $N$ is a product manifold. We show
that when $N=S^2\times S^1$, the minimum of $W(T^2)$ is 0, and when
$N=R^2\times S^1$, there exists no torus having least Willmore functional. When
$N=H^2(-c)\times S^1$, and $x=\gamma\times S^1$, the minimum of $W(x)$ is
$2\pi^2\sqrt{c}$.
