  Thermalization in an isolated oscillating Bose-Einstein condensate in a
disordered trap is investigated. We show Shannon entropy in $x$ or $p$
representation is the eligible one to describe the thermalization. Besides, we
show that multiple scattering with the disorder generates more and more
incoherent thermal particles and condensed and thermal particles act as mutual
heat bath that results in the thermalization of the whole system. We also
demonstrate that Loschmidt's paradox can be resolved in the present system.
