Abstract
Nematic phase transition with both uniaxial and biaxial order parameters is studied on basis of Maier-Saupe model in the two kind of external fields which are conjugated to the order parameters, respectively. A global phase diagram in fields versus temperature space for bulk is obtained in the mean field theory, which is similar to the phase diagram of the three-state Potts model in three dimension. The phase diagram in the uniaxial field which is a cross section of the global one is applied to the phase transition at the thin systems with homeotropic and planer anchoring walls.
