  Let $R=k[x_1,...,x_n]$ be the polynomial ring in $n$ variables over a field
$k$ and $I$ be a matroidal ideal of degree $d$. In this paper, we study the
unmixedness properties and the arithmetical rank of $I$. Moreover, we show that
$ara(I)=n-d+1$. This answer to the conjecture that made by H. J. Chiang-Hsieh
\cite[Conjecture]{C}.
