  We present modelling of line polarization to study multi-dimensional geometry
of stripped-envelope core-collapse supernovae (SNe). We demonstrate that a
purely axisymmetric, two-dimensional geometry cannot reproduce a loop in the
Stokes Q-U diagram, i.e., a variation of the polarization angles along the
velocities associated with the absorption lines. On the contrary,
three-dimensional (3D) clumpy structures naturally reproduce the loop. The fact
that the loop is commonly observed in stripped-envelope SNe suggests that SN
ejecta generally have a 3D structure. We study the degree of line polarization
as a function of the absorption depth for various 3D clumpy models with
different clump sizes and covering factors. Comparison between the calculated
and observed degree of line polarization indicates that a typical size of the
clump is relatively large, >~ 25 % of the photospheric radius. Such large-scale
clumps are similar to those observed in the SN remnant Cassiopeia A. Given the
small size of the observed sample, the covering factor of the clumps is only
weakly constrained (~ 5-80 %). The presence of large-scale clumpy structure
suggests that the large-scale convection or standing accretion shock
instability takes place at the onset of the explosion.
