Root vectors are important
to understand quantized enveloping algebras. In this paper we establish a
commutation formula for root vectors. By means of the formula we show that
particular orders on root system are not necessary in constructing some
integral bases of a quantized enveloping algebra (Theorem 2.4). Moreover using
the formula we can show that certain PBW bases are orthogonal bases of
the bilinear form considered by Kashiwara in his work on crystal bases, see
3.9.