  We investigated thermal leptogenesis scenarios in the left-right symmetric
extension of the standard model. In the SO(10) GUT framework, we impose the
D-parity realization below GUT scale. These two conditions makes our model more
restrictive and predictive. In such a case, a D-parity odd singlet in the
4-index antisymmetric tensor representation of SO(10) have a critical role.
This singlet have prospects of causing a very large mass hierarchy between
SU(2)L and SU(2)R triplet scalars.
  We test our model by computing baryogenesis via leptogenesis. The heavy
right-handed neutrinos N's and the SU(2)L triplet scalar can generate the
lepton number asymmetry. Leptogenesis scenarios can be categorized by these
mass scales. If the light neutrinos are Majorana and have a hierarchical mass
spectrum, we can obtain a successful result in leptogenesis through lightest
N-decay. But we found that the normal mass hierarchy of the light neutrinos
conflicts with leptogenesis through triplet-decay.
