Up to now, poly-para-phenylene (PPP) can be considered as the stiffest and strongest polymer under ambient temperature conditions. As an amorphous thermoplastic, a thermal treatment will not change the material's morphology, but may influence its inner free volume (physical aging) and corresponding mechanical properties. This was investigated in this study with regard to the hardness and fracture toughness of PPP. It turned out that under the thermal conditions chosen, only moderate effects on these properties could be detected. Besides it was of interest how PPP compares in these properties relative to other advanced thermoplastics (neat and short glass fiber reinforced). Various features, such as striations and parabolic holes, on the fracture surfaces of PPP were also discussed.