  Let $\mathbf{U}$ be the quantized enveloping algebra and $\dot{\mathbf{U}}$
its modified form. Lusztig gives some symmetries on $\mathbf{U}$ and
$\dot{\mathbf{U}}$. Since the realization of $\mathbf{U}$ by the reduced
Drinfeld double of the Ringel-Hall algebra, one can apply the BGP-reflection
functors to the double Ringel-Hall algebra to obtain Lusztig's symmetries on
$\mathbf{U}$ and their important properties, for instance, the braid relations.
In this paper, we define a modified form $\dot{\mathcal{H}}$ of the Ringel-Hall
algebra and realize the Lusztig's symmetries on $\dot{\mathbf{U}}$ by applying
the BGP-reflection functors to $\dot{\mathcal{H}}$.
