  Let $l_{n}$ be the length of the $n$-th instability interval of the Hill
operator $Ly=-y^{\prime\prime}+q(x)y$. We obtain that if $l_{n}=o(n^{-2})$ then
$c_{n}=o(n^{-2})$, where $c_{n}$ are the Fourier coefficients of $q$. Using
this inverse result, we prove: Let $l_{n}=o(n^{-2})$. If $\{(n\pi)^{2}:
\textrm{$n$ even and $n>n_{0}$}\}$ is a subset of the periodic spectrum of Hill
operator then $q=0$ a.e., where $n_{0}$ is a positive large number such that
$l_{n}<\varepsilon n^{-2}$ for all $n>n_{0}(\varepsilon)$ with some
$\varepsilon>0$. A similar result holds for the anti-periodic case.
