  If reheating of the Universe takes place via Planck-suppressed decay, it
seems that the thermalization of produced particles might be delayed, since
they have large energy/small number densities and number violating large angle
scatterings which decrease the momentum of particles by large amount are
inefficient correspondingly. In this paper, we study the thermalization of such
"under occupied" decay products in detail, following recent developments in
understanding the thermalization of non-abelian plasma. Contrary to the above
naive expectation, it is shown that in most cases thermalization after/during
reheating occurs instantaneously by properly taking account of scatterings with
small angles and of particles with small momenta. In particular, the condition
for instantaneous thermalization before the completion of reheating is found to
be $\alpha^{8/5} \gg (m_\phi / M_{\rm pl}) (M_{\rm pl}^2 \Gamma_\phi /
m_\phi^3)^{1/5}$, which is much milder than that obtained in previous works
with small angle scatterings taken into account.
