  The polarization measurement is an important tool to probe the prompt
emission mechanism in gamma-ray bursts (GRBs). The synchrotron photons can be
scattered by cold electrons in the outflow via Compton scattering processes.
The observed polarization depends on both the photon energy and the viewing
angle. With the typical bulk Lorentz factor $\Gamma \sim 200$, photons with
energy $E>10$ MeV tend to have smaller polarization than photons with energy
$E<1$ MeV. At the right viewing angle, i.e. $\theta \sim \Gamma^{-1}$, the
polarization achieves its maximal value, and the polarization angle changes
$90^{\circ}$ relative to the initial polarization direction. Thus, the
synchrotron radiation plus Compton scattering model can naturally explain the
$90^{\circ}$ change of the polarization angle in GRB 100826A.
