  We assemble a sample consisting of 66 active galactic nuclei (AGNs) from
literature and the {\em XMM-Newton} archive in order to investigate the origin
of the 6.4 keV narrow iron K$\alpha$ line (NIKAL). The X-ray Baldwin effect of
the NIKAL is confirmed in this sample. We find the equivalent width ($EW$) of
the NIKAL is more strongly inversely correlated with Eddington ratio ($\ce$)
than the 2-10 keV X-ray luminosity. Our sample favors the origin from the dusty
torus and the X-ray Baldwin effect is caused by the changing opening angle of
the dusty torus. The relation $EW-\ce$ can be derived from a toy model of the
dusty torus. If the unification scheme is valid in all AGNs, we can derive the
Baldwin effect from the fraction of type {\sc ii} AGNs to the total population
given by {\em Chandra} and {\em Hubble} deep surveys. Thus the evolution of
populations could be reflected by the NIKAL's Baldwin effect.
