\documentclass{jarticle}
\usepackage{array}

\newcolumntype{C}[1]{>{\centering}p{#1\linewidth}|}

\begin{document}

\begin{tabular}{|C{.2}C{.4}}
\hline
item 1 & item 2\tabularnewline
\hline 
1&2\tabularnewline
\hline
\end{tabular}

\end{document}
