  We figure out the famous Klein's paradox arising from the reflection problem
when a Dirac particle encounters a step potential with infinite width. The key
is to piecewise solve Dirac equation in such a way that in the region where the
particle's energy E is greater (less) than the potential V, the solution of the
positive (negative) energy branch is adopted. In the case of Klein-Gordon
equation with a piecewise constant potential, the equation is decoupled to
positive and negative energy equations, and reflection problem is solved in the
same way. Both infinitely and finitely wide potentials are considered. The
reflection coefficient never exceeds 1. The results are applied to discuss the
transmissions of particles with no mass or with very small mass.
