  The high efficiency of converting kinetic energy into gamma-rays estimated
with late-time afterglows in Gamma-Ray Burst (GRB) phenomenon challenges the
commonly accepted internal-shock model. However, the efficiency is still highly
uncertain because it is sensitive to many effects. In this Letter we study the
sideways expansion effect of jets on estimating the efficiency. We find that
this effect is considerable, reducing the efficiency by a factor of $\sim0.5$
for typical parameters, when the afterglow data $\sim 10$ hr after the GRB
trigger are used to derive the kinetic energy. For a more dense circumburst
medium, this effect is more significant. As samples, taking this effect into
account, we specifically calculate the efficiency of two bursts whose
parameters were well constrained. Almost the same results are derived. This
suggests that the sideways expansion effect should be considered when the GRB
efficiency is estimated with the late afterglow data.
