  We present a theory to describe domain formation observed very recently in a
quenched Rb-87 gas, a typical ferromagnetic spinor Bose system. An overlap
factor is introduced to characterize the symmetry breaking of M_F=\pm 1
components for the F=1 ferromagnetic condensate. We demonstrate that the domain
formation is a co-effect of the quantum coherence and the thermal relaxation. A
thermally enhanced quantum-oscillation is observed during the dynamical process
of the domain formation. And the spatial separation of domains leads to
significant decay of the M_F=0 component fraction in an initial M_F=0
condensate.
