We prove that a Kreiss bounded $$C_0$$
C
0
semigroup $$(T_t)_{t \ge 0}$$
(
T
t
)
t
≥
0
on a Hilbert space has asymptotics $$\left\| T_t\right\| = {\mathcal{O}}\big (t/\sqrt{\log (t)}\big ).$$
T
t
=
O
(
t
/
log
(
t
)
)
.
Then, we give an application to perturbed wave equation.