  We study jet substructures of a boosted polarized top quark, which undergoes
the semileptonic decay $t\to b\ell\nu$, in the perturbative QCD framework. The
jet mass distribution (energy profile) is factorized into the convolution of a
hard top-quark decay kernel with the bottom-quark jet function (jet energy
function). Computing the hard kernel to leading order in QCD and inputting the
latter functions from the resummation formalism, we observe that the jet mass
distribution is not sensitive to the helicity of the top quark, but the energy
profile is: energy is accumulated faster within a left-handed top jet than
within a right-handed one, a feature related to the $V-A$ structure of weak
interaction. It is pointed out that the energy profile is a simple and useful
jet observable for helicity discrimination of a boosted top quark, which helps
identification of physics beyond the Standard Model at the Large Hadron
Collider. The extension of our analysis to other jet substructures, including
those associated with a hadronically decaying polarized top quark, is proposed.
