\documentclass[12pt]{article}
\pagestyle{empty}
\begin{document}
\begin{displaymath}
P_{\rm accept} ({\bf r}_m, ..., {\bf r}_z)
 = {\rm Min}\left[1,
{
{\prod_{k=1}^{n_{\rm step}} W_{k}^{\rm new}}
\over
{\prod_{k=1}^{n_{\rm step}} W_{k}^{\rm old}}
}
\times
{
{\prod_{s=m}^{z} w_{\rm bias}({{\bf r}_s},{\bf r}_{\rm exist})^{\rm old}}
\over
{\prod_{s=m}^{z} w_{\rm bias}({{\bf r}_s},{\bf r}_{\rm exist})^{\rm new}}
}
\right]
\end{displaymath}
\end{document}