  We propose a resonant leptogenesis scenario in a U(1)_{B-L} gauge extension
of the standard model to generate large lepton asymmetries for cosmological
baryon asymmetry and dark matter. After B-L number is spontaneously broken,
inflaton can pick up a small vacuum expectation value for the mass splits of
three pairs of quasi-degenerately heavy Majorana neutrinos and the masses of
three sterile neutrinos. With thermal mass effects of sphalerons, the observed
small baryon asymmetry can be converted from large lepton asymmetries of
individual flavors although total lepton asymmetry is assumed zero. The mixing
between sterile and active neutrinos is elegantly suppressed by the heavy
Majorana neutrinos. Before the active neutrinos start their strong flavor
conversions, the sterile neutrinos as warm dark matter can be produced by
resonant active-sterile neutrino oscillations to reconcile X-ray and
Lyman-\alpha bounds. Small neutrino masses are naturally realized by seesaw
contributions from the heavy Majorana neutrinos and the sterile neutrinos.
