  Given a Calder\'{o}n--Zygmund (C--Z for short) operator $T$, which satisfies
H\"ormander condition, we prove that: if $T$ maps all the characteristic atoms
to $WL^{1}$, then $T$ is continuous from $L^{p}$ to $L^{p}(1<p<\infty)$. So the
study of strong continuity on arbitrary function in $L^{p}$ has been changed
into the study of weak continuity on characteristic functions.
