  Linear polarization have been observed in both the prompt phase and afterglow
of some bright gamma-ray bursts (GRBs). Polarization in the prompt phase spans
a wide range, and may be as high as $\gtrsim 50\%$. In the afterglow phase,
however, it is usually below $10\%$. According to the standard fireball model,
GRBs are produced by synchrotron radiation and Compton scattering process in a
highly relativistic jet ejected from the central engine. It is widely accepted
that prompt emissions occur in the internal shock when shells with different
velocities collide with each other, and the magnetic field advected by the jet
from the central engine can be ordered in large scale. On the other hand,
afterglows are often assumed to occur in the external shock when the jet
collides with interstellar medium, and the magnetic field produced by the shock
through, e.g. Weibel instability, is possibly random. In this paper, we
calculate the polarization properties of the synchrotron self-Compton process
from a highly relativistic jet, in which the magnetic field is randomly
distributed in the shock plane. We also consider the generalized situation
where a uniform magnetic component perpendicular to the shock plane is
superposed on the random magnetic component. We show that, the polarization is
hardly to be larger than $10\%$ if the seed electrons are isotropic in the jet
frame. This may account for the observed upper limit of polarization in the
afterglow phase of GRBs. In addition, if the random and uniform magnetic
components decay with time in different speeds, then the polarization angle may
change $90^{\circ}$ duration the temporal evolution.
