\documentclass{article}
\usepackage{slashbox}
\title{slashbox -- pakietomat.wordpress.com}
\author{}
\date{\relax}
\begin{document}
\begin{tabular}{|l|l|l|} \hline
\backslashbox{ABC}{DEF} & 1 & 2 \\ \hline \hline
E & 3 & 4 \\ \hline
F & 5 & 6 \\ \hline
\end{tabular}
\end{document}
