  Whether gamma-ray bursts are highly beamed or not is a very important
question, since it has been pointed out that the beaming will lead to a sharp
break in the afterglow light curves during the ultra-relativistic phase, with
the breaking point determined by $\Gamma \sim 1/\theta_{0}$, where $\Gamma$ is
the bulk Lorentz factor and $\theta_{0}$ is the initial half opening angle of
the ejecta, and such a break is claimed to be present in the light curves of
some GRBs. In this paper we will examine whether all the observed breaks in GRB
afterglow light curves can be explained by jet effects. Here we present a
detailed calculation of the jet evolution and emission, and have obtained a
simple formula of bulk Lorentz factor evolution. We show that the light curves
are very smoothly steepened by jet effect, and the shape of the light curve is
determined by only one parameter -- $(E/n)^{1/8} \theta_{0}^{3/4}$, where E and
n are the fireball energy and surrounding medium density. We find that for
GRB990123 and GRB991216, the jet model can approximately fit their light
curves, and the values of $(E_{51}/n_1)^{1/8} \theta_{0}^{3/4}$ are about 0.17
and 0.22 respectively. While for GRB990510, GRB000301c, GRB000926 and
GRB010222, their light curves cannot be fitted by the jet model, which suggests
that the breaks may be caused by some other reasons, jet should be not the
unique reason.
