  X-ray flares, lasting for $\sim 100 - 1000$ s in the X-ray band, are often
observed following gamma-ray bursts (GRBs). The physical origin of X-ray flares
is still unknown merely with the temporal/spectral information. On the other
hand, some polarimeters are expected to be launched within several years thanks
to the increasing interest on astronomical X-ray polarimetry. Here, by assuming
that X-ray flares are synchrotron radiation from relativistic spherical shells,
we show that the linear polarization degree during the rising phase of an X-ray
flare is much higher for the emitting region with toroidal magnetic fields than
that with random magnetic fields. In the decay phase of the flare, the
evolution of the polarization degree is determined by the curvature effect of
the emitting shell, which is a natural feature of jet scenarios for flares.
Therefore, the measurement of the polarization of X-ray flares would provide a
useful tool to probe the configuration of magnetic fields in the emission
region, and may even help to test the curvature effect. The information on the
magnetic configuration can further help us to understand the properties of GRB
jets.
