  We present time-resolved spectral analysis of the steep decay segments of 29
bright X-ray flares of gamma-ray bursts (GRBs) observed with the Swift/X-ray
telescope, and model their lightcurves and spectral index evolution behaviors
with the curvature effect model. Our results show that the observed rapid flux
decay and strong spectral index evolution with time can be well fit with this
model, and the derived characteristic timescales ($t_c$) are in the range of
$33\sim 264$ seconds. Using an empirical relation between the peak luminosity
and the Lorentz factor derived from the prompt gamma-rays, we estimate the
Lorentz factors of the flares ($\Gamma_{\rm X}$). We obtain $\Gamma_{\rm
X}=17\sim 87$ with a median value of $52$, which is smaller than the initial
Lorentz factors of prompt gamma-ray fireballs. With the derived $t_c$ and
$\Gamma_{\rm X}$, we constrain the radiating regions of 13 X-ray flares,
yielding $R_{\rm X}=(0.2\sim 1.1)\times 10^{16}$ cm, which are smaller than the
radii of the afterglow fireballs at the peak times of the flares. A long
evolution feature from prompt gamma-ray phase to the X-ray epoch is found by
incorporating our results with a sample of GRBs whose initial Lorentz factors
are available in literatures, i.e., $\Gamma\propto [t_{p}/(1+z)]^{-0.69\pm
0.06}$. These results may shed lights on the long term evolution of GRB central
engines.
