  Due to some refinements in the dynamics, we can follow the overall evolution
of a realistic jet numerically till its bulk velocity being as small as $\beta
c \sim 10^{-3} c$. We find no obvious break in the optical light curve during
the relativistic phase itself. However, an obvious break does exist at the
transition from the relativistic phase to the non-relativistic phase, which
typically occurs at time $t \sim 10^6$ --- $10^{6.5}$ s (i.e., 10 --- 30 d).
The break is affected by many parameters, such as the electron energy fraction
$\xi_{\rm e}$, the magnetic energy fraction $\xi_{\rm B}^2$, the initial half
opening angle $\theta_0$, and the medium number density $n$. Increase of any of
them to a large enough value will make the break disappear. Although the break
itself is parameter-dependent, afterglows from jetted GRB remnants are
uniformly characterized by a quick decay during the non-relativistic phase,
with power law timing index $\alpha \geq 2.1$. This is quite different from
that of isotropic fireballs, and may be of fundamental importance for
determining the degree of beaming in $\gamma$-ray bursts observationally.
