\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{units}
\usepackage{ifthen}
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}
\tikzstyle{vertex}=[draw,black,fill=blue,circle,minimum size=10pt,inner sep=0pt]
\tikzstyle{edge}=[very thick]
\begin{tikzpicture}[scale=2.5]
    \node (a)[vertex,fill=gray!10,align=left] at (0,0) {S};
    \node (b)[vertex,fill=gray!10,align=left] at (1,0)  {R};

    \path[thick,->,bend left] (a)    edge node [anchor=center,above,sloped] {$\nicefrac{2}{10}$} (b);
    \path[thick,->,bend left] (b)    edge node [anchor=center,below,sloped] {$\nicefrac{4}{10}$} (a);
    \path[thick,->] (a)    edge[loop left, looseness=15] node [anchor=center,above] {$\nicefrac{8}{10}$} (a);
    \path[thick,->] (b)    edge[loop right, looseness=15] node [anchor=center,above] {$\nicefrac{6}{10}$} (b);
\end{tikzpicture}
\end{document}
