\documentclass{article}
\usepackage[utf8]{inputenc}

\usepackage{tikz}

\begin{document}

\begin{tikzpicture}
\node[circle, draw=black, minimum size=10mm,label={[font=\small,text=blue]45:{0}}] {I}
    child {node[circle, draw=black, minimum size=10mm] {if}}
    child {node(0)[circle, draw=black, minimum size=10mm,label={[font=\small,text=blue]45:{1}}] {B}
        child {node(1)[circle, draw=black, minimum size=10mm] {$b_1$}}
    }
    child {node[circle, draw=black, minimum size=10mm] {then}}
    child {node[circle, draw=black, minimum size=10mm,label={[font=\small,text=blue]45:{2}}] {C}
        child {node[circle, draw=black, minimum size=10mm] {$c_2$}}
    }
    ;
\end{tikzpicture}

\end{document}