Equations aligned center

If you want to have each equation centered rather than aligned on some specific point (such as the first = symbol), you should use a gathered environment instead of an aligned environment.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation} \label{eq:test}
\begin{gathered}
A=AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\\
B=BBBBBBBBBBBBBBBB\\
C=CCCCC, D=DDDDDDD\\
E=EEEEE, F=FFFFFFF
\end{gathered}
\end{equation}
\end{document} 

Edit: I automatically assumed that you wanted all the equations aligned on the = sign. The right answer to center them is Mico's. However, this is another possibility that you may try.

You only need to tell Latex how to align your equations, with the & symbol:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation} \label{eq:test}
    \begin{aligned} 
        A &= AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA\\
        B &= BBBBBBBBBBBBBBBB\\
        C &= CCCCC, D=DDDDDDD\\    
        E &= EEEEE, F=FFFFFFF
    \end{aligned}
\end{equation}

\end{document}