In this work the effect of the compatibility between organoclays and styrene on the flammability of polystyrene/clay nanocomposites obtained through in situ incorporation was investigated. The reactions were carried out by bulk polymerization. The compatibility between organoclays and styrene was inferred from swelling of the organoclay in styrene. The nanocomposites were characterized by X-ray diffraction and Transmission Electron Microscopy. The heat release rate was obtained by Cone Calorimeter and the nanocomposites were tested by UL94 horizontal burn test. Results showed that intercalated and partially exfoliated polystyrene/clay nanocomposites were obtained depending on the swelling behavior of the organoclay in styrene. The nanocomposites submitted to UL94 burning test presented a burning rate faster than the virgin polystyrene (PS), however an increase of the decomposition temperature and an accentuated decrease on the peak of heat release of the nanocomposites were also observed compared to virgin PS. These results indicate that PS/clay nanocomposites, either intercalated or partially exfoliated, reduced the flammability approximately by the same extent, although reduced the ignition resistance of the PS.