  The condition for adiabatic approximation are of basic importance for the
applications of the adiabatic theorem. The traditional quantitative condition
was found to be necessary but not sufficient, but we do not know its physical
meaning and the reason why it is necessary from the physical point of view. In
this work, we relate the adiabatic theorem to the uncertainty relation, and
present a clear physical picture of the traditional quantitative condition. It
is shown that the quantitative condition is just the amplitude of the
probability of transition between two levels in the time interval which is of
the order of the time uncertainty of the system. We also present a new
sufficient condition with clear physical picture.
