  In the unification scheme of Seyfert galaxies, a dusty torus blocks the
continuum source and broad line region in Seyfert 2 galaxies. However it is not
clear whether or not and to what extent the torus affects the narrow line
spectra. In this paper, we show that Seyfert 1 and Seyfert 2 galaxies have
different distributions on the [OIII]/H$\beta $ vs [NII]/H$\alpha$ diagram (BPT
diagram) for narrow lines. Seyfert 2 galaxies display a clear left boundary on
the BPT diagram and only 7.3% of them lie on the left. By contrast, Seyfert 1
galaxies do not show such a cutoff and 33.0% of them stand on the left side of
the boundary. Among Seyfert 1 galaxies, the distribution varies with the
extinction to broad lines. As the extinction increases, the distribution on BPT
diagram moves to larger [NII]/H$\alpha$ value. We interpret this as an evidence
for the obscuration of inner dense narrow line region by the dusty torus. We
also demonstrate that the [OIII] and broad line luminosity correlation depends
on the extinction of broad lines in the way that high extinction objects have
lower uncorrected [OIII] luminosities, suggesting that [OIII] is partially
obscured in these objects. Therefore, using [OIII] as an indicator for the
nuclear luminosity will systematically under-estimate the nuclear luminosity of
Seyfert 2 galaxies.
