  Total-Routhian-Surface calculations have been performed to investigate the
shape evolutions of $A\sim80$ nuclei, $^{80-84}$Zr, $^{76-80}$Sr and
$^{84,86}$Mo. Shape coexistences of spherical, prolate and oblate deformations
have been found in these nuclei. Particularly for the nuclei, $^{80}$Sr and
$^{82}$Zr, the energy differences between two shape-coexisting states are less
than 220 keV. At high spins, the $g_{9/2}$ shell plays an important role for
shape evolutions. It has been found that the alignment of the $g_{9/2}$
quasi-particles drives nuclei to be triaxial.
