  X-ray flares were discovered in the afterglow phase of gamma-ray bursts
(GRBs) by the {\em Swift} satellite a decade ago and known as a canonical
component in GRB X-ray afterglows. In this paper, we constrain the Lorentz
factors of GRB X-ray flares using two different methods. For the first method,
we estimate the lower limit on the bulk Lorentz factor with the flare duration
and jet break time. In the second method, the upper limit on the Lorentz factor
is derived by assuming that the X-ray flare jet has undergone saturated
acceleration. We also re-estimate the initial Lorentz factor with GRB afterglow
onsets, and find the coefficient of the theoretical Lorentz factor is 1.67
rather than the commonly used 2 for interstellar medium (ISM) and 1.44 for the
wind case. We find that the correlation between the limited Lorentz factor and
the isotropic radiation energy of X-ray flares in the ISM case is more
consistent with that of prompt emission than the wind case in a statistical
sense. For a comparison, the lower limit on Lorentz factor is statistically
larger than the extrapolation from prompt bursts in the wind case. Our results
indicate that X-ray flares and prompt bursts are produced by the same physical
mechanism.
