  Using Brownian dynamics simulation, we study the orientational order in
melting transition of colloidal systems with $'$soft$'$ Yukawa potential. The
bond-orientational order parameter $\Phi_{6}$ and the bond-orientational order
function $g_B(r)$ are calculated in two-dimensional systems. It is found that a
two-stage transition and the hexatic phase are indeed existent in
two-dimensional melitng, which is consistent with the prediction of the
Kosterlitz-Thouless-Halperin-Nelson-Young theory. For comparing with the
melting process in three-dimensional systems, the probability distribution of
single-particle local order parameter is introduced. Based on the extensive
simulations, it is qualitatively suggested that the breakdown of local order
only occurs on the fractional part of the colloidal systems for the
two-dimensional melting, but in three-dimensional melting, this breakdown takes
place on the whole systems at the same time.
