  We study vector bundles on flag varieties over an algebraically closed field
$k$. In the first part, we suppose $G=G_k(d,n)$ $(2\le d\leq n-d)$ to be the
Grassmannian manifold parameterizing linear subspaces of dimension $d$ in
$k^n$, where $k$ is an algebraically closed field of characteristic $p>0$. Let
$E$ be a uniform vector bundle over $G$ of rank $r\le d$. We show that $E$ is
either a direct sum of line bundles or a twist of a pull back of the universal
bundle $H_d$ or its dual $H_d^{\vee}$ by a series of absolute Frobenius maps.
In the second part, splitting properties of vector bundles on general flag
varieties $F(d_1,\cdots,d_s)$ in characteristic zero are considered. We prove a
structure theorem for bundles over flag varieties which are uniform with
respect to the $i$-th component of the manifold of lines in
$F(d_1,\cdots,d_s)$. Furthermore, we generalize the
Grauert-M$\ddot{\text{u}}$lich-Barth theorem to flag varieties. As a corollary,
we show that any strongly uniform $i$-semistable $(1\le i\le n-1)$ bundle over
the complete flag variety splits as a direct sum of special line bundles.
