  We intend to determine the type of circumburst medium and measure directly
the initial Lorentz factor $\Gamma_0$ of GRB outflows. If the early X-ray
afterglow lightcurve has a peak and the whole profile across the peak is
consistent with the standard external shock model, the early rise profile of
light curves can be used to differentiate whether the burst was born in
interstellar medium (ISM) or in stellar wind. In the thin shell case, related
to a sub-relativistic reverse shock, the peak time occurring after the end of
the prompt emission, can be used to derive an accurate $\Gamma_0$, especially
for the ISM case. The afterglow lightcurves for a flat electron spectrum
$1<p<2$ have been derived analytically. In our GRB sample, we obtain $\Gamma_0
\sim 300$ for the bursts born in ISM. We did not find any good case for bursts
born in stellar wind and behaving as a thin shell that can be used to constrain
$\Gamma_0$ reliably.
