Assuming a simple biaxial molecular interaction model in nematic liquid crystal, and using the mean field approximation, phase transitions from isotropic to uniaxial nematics, uniaxial to biaxial nematics are obtained. The result obtained shows that at certain temperatures, the biaxial phases are induced by the asymmetry of molecular shape of liquid crystal. In nuclear magnetic resonance experiment, a general relation between order parameters and spectrum frequency splitting is derived, and possible experimental measurement is discussed.