  We have investigated the ensemble regularities of the equivalent widths (EWs)
of MgII 2800 emission line of active galactic nuclei (AGNs), using a uniformly
selected sample of 2092 Seyfert 1 galaxies and quasars at 0.45 <= z <= 0.8 in
the spectroscopic data set of Sloan Digital Sky Survey Fourth Data Release. We
find a strong correlation between the EW of MgII and the AGN Eddington ratio
(L/L_Edd): EW(MgII) \propto (L/L_Edd)^{-0.4}. Furthermore, for AGNs with the
same L/L_Edd, their EWs of MgII show no correlation with luminosity, black hole
mass or line width, and the MgII line luminosity is proportional to continuum
luminosity, as expected by photoionization theory. Our result shows that MgII
EW is not dependent on luminosity, but is solely governed by L/L_Edd.
