  Recent study on the charged top-pion correction to R_b shows that it is
negative and large, so that the precision experimental value of R_b gives rise
to a severe constraint on the topcolor-assisted technicolor models such that
the top-pion mass should be of the order of 1 TeV. In this paper, we restudy
this constraint by further taking account the extended technicolor gauge boson
correction which is positive. With this positive contribution to R_b, the
constraint on the topcolor-assisted technicolor models from R_b changes
significantly. The top-pion mass is allowed to be in the region of a few
hundred GeV depending on the models.
