\usetikzlibrary{arrows}
\begin{tikzpicture}[x=1cm, y=1cm,->,auto,>=stealth',thick
,c/.style={rectangle,fill=green!20,draw}]
\path (0,1) node (entry) [coordinate] {};
\path (-1,0) node (c0) [c] {C0};
\path (1,0) node (c1) [c] {C1};
\path (0,-1) node (exit) [c] {B};
\path
(0,2) edge (entry)
(entry) edge (c0)
(c0) edge (exit)
(exit) edge (c1)
(c1) edge (entry)
(exit) edge (0,-2)
;
\end{tikzpicture}