We study the dynamical instability against bar-mode deformation of differentially rotating stars .
We performed numerical simulation and linear perturbation analysis adopting polytropic equations of state with the polytropic index n = 1 .
It is found that rotating stars of a high degree of differential rotation are dynamically unstable even for the ratio of the kinetic energy to the gravitational potential energy of O ( 0.01 ) .
Gravitational waves from the final nonaxisymmetric quasistationary states are calculated in the quadrupole formula .
For rotating stars of mass 1.4 M _ { \odot } and radius several 10 km , gravitational waves have frequency several 100 Hz and effective amplitude \sim 5 \times 10 ^ { -22 } at a distance of \sim 100 Mpc .