  X-ray flares of gamma-ray bursts (GRBs) are usually observed in the soft
X-ray range and the spectral coverage is limited. In this paper, we present an
analysis of 32 GRB X-ray flares that are simultaneously observed by both BAT
and XRT on board the Swift mission, so a joint spectral analysis with a wider
spectral coverage is possible. Our results show that the joint spectra of 19
flares are fitted with the absorbed single power-law or the Band function
models. More interestingly, the joint spectra of the other 13 X-ray flares are
fitted with the absorbed single power-law model plus a black body (BB)
component. Phenomenally, the observed spectra of these 13 flares are analogous
to several GRBs with a thermal component, but only with a much lower
temperature of $kT=1\sim 3$ keV. Assuming that the thermal emission is the
photosphere emission of the GRB fireball, we derive the fireball properties of
the 13 flares that have redshift measurements, such as the bulk Lorentz factor
$\Gamma_{\rm ph}$ of the outflow. The derived $\Gamma_{\rm ph}$ range from $50$
to $150$ and a relation of $\Gamma_{\rm ph}$ to the thermal emission luminosity
is found. It is consistent with the $\Gamma_0-L_{\rm iso}$ relation that are
derived for the prompt gamma-ray emission. We discuss the physical implications
of these results within the content of jet composition and radiation mechanism
of GRBs and X-ray flares.
