  The jet composition and energy dissipation mechanism of Gamma-ray bursts
(GRBs) and Blazars are fundamental questions which remain not fully understood.
One plausible model is to interpret the $\gamma$-ray emission of GRBs and
optical emission of blazars as synchrotron radiation of electrons accelerated
from the collision-induced magnetic dissipation regions in
Poynting-flux-dominated jets. The polarization observation is an important and
independent information to test this model. Based on our recent 3D relativistic
MHD simulations of collision-induced magnetic dissipation of magnetically
dominated blobs, here we perform calculations of the polarization properties of
the emission in the dissipation region and apply the results to model the
polarization observational data of GRB prompt emission and blazar optical
emission. We show that the same numerical model with different input parameters
can reproduce well the observational data of both GRBs and blazars, especially
the $90^{\circ}$ polarization angle (PA) change in GRB 100826A and the
$180^{\circ}$ PA swing in Blazar 3C279. This supports a unified model for GRB
and blazar jets, suggesting that collision-induced magnetic reconnection is a
common physical mechanism to power the relativistic jet emission from events
with very different black hole masses.
