  Einstein-Podolsky-Rosen's paper in 1935 is discussed in parallel with an EPR
experiment on $K^0\bar{K}^0$ system in 1998, yielding a strong hint of
distinction in both wave-function and operators between particle and
antiparticle at the level of quantum mechanics (QM). Then it is proposed that
the CPT invariance in particle physics leads naturally to a basic postulate
that the (newly defined) space-time inversion (${\bf x}\to -{\bf x},t\to -t$)
is equivalent to the transformation between particle and its antiparticle. The
evolution of this postulate from nonrelativistic QM via relativistic QM till
the quantum field theory is discussed in some detail. The Klein paradox for
both Klein-Gordon equation and Dirac equation is also discussed.
  Keywords: CPT invariance, Antiparticle, Quantum mechanics, Quantum field
theory
