The application of 7a,14c-dihydronaphtho[2,1-b]naphtho[2’,1’:4,5]furo[3,2-d]furan (FF) was investigated as an effective stabilizer for polystyrene (PS) at higher temperatures. PS containing 1% FF retained 97.9% of its tensile strength (Ts) after beat treatment at 190 degrees C for 90 min in the absence of oxygen and 95% of Ts heated at 160 degrees C for 150 min in air and molded at 160 degrees C for 90 min under reduced pressure, whereas PS without stabilizers lost Ts by 50% under the former treated condition and 20% under the latter one. The effectiveness of FF as a heat stabilizer for PS was confirmed by thermal analyses (TG and DSC), in air and in nitrogen. A possible stabilization mechanism was studied by IR spectroscopy and MO calculation. formation of a stable radical from FF was observed by ESR spectroscopy.