
\documentclass{article}
  \usepackage{fancybox}
  \pagestyle{empty}

\begin{document}
\linethickness{3pt}
\Ovalbox{
\begin{Beqnarray}
ds^2&\,=\,&
\displaystyle{\frac{\epsilon^{\prime 2}}{L^2}
\frac{12N}{(N+1)(N+2)}}
\end{Beqnarray}
}
\end{document}
