The different organic montmorillonite (OMMT) orientation HIPS/OMMT nanocomposites were prepared through the melting intercalated method and second produce. The flame retardancy of the two composites was evaluated by the Cone calorimeter. The structure of burnt residues was observed by digital camera and scanning electron microscope (SEM). The experimental results show that the Heat Release Rate (HRR) and Mass Loss Rate (MLR) for horizontal orientation HIPS/OMMT nanocomposites are lowed in comparison with the vertical orientation composite. The burnt residue of horizontal orientation HIPS/OMMT nanocomposites has a continuous skin layer and three-dimensional texture cellular layer under the skin layer after burning. This kind of structure of char residues has a great effect on the flame retardancy of the composites. While vertical HIPS/OMMT nanocomposites do not formed the continuous skin layer, and the voids in char layer are vertical, which provide the passage for flammable gas, and its flame retardancy is not good.