In the current study, the effects of four different POSS nanoparticles on the flame retardant properties of intumescent PP were investigated through mass-loss calorimetry (MLC), limiting oxygen index (LOI) test and UL-94 ratings. In addition, the structure-property relationship was enlightened via thermal, morphological and mechanical tests. The four different POSS types, namely aminopropyl isobutyl-POSS (A-POSS), octaphenyl-POSS (OP-POSS), octaammonium-POSS (OA-POSS) and trissulfonic acid propyl-POSS (TS-POSS), were used. The ratio of ammonium polyphosphate (APP) to pentaerythritol (PER) was kept constant and the POSS content was varied as 0.5%, 1% and 3%. The MLC, LOI and UL-94 tests revealed that the addition of POSS particles had adjuvant on the flame retardant properties of PP/IFR system. All POSS nanoparticles showed their highest adjuvant effect at the lowest concentration of 0.5 wt%. The highest UL-94 rating and the highest LOI values were achieved in the presence of OA-POSS and TS-POSS. The ceramic layer formation was monitored using scanning electron microscopy (SEM) and FTIR on the char residues. The thermal properties of PP/IFR/POSS composites were obtained via differential scanning calorimeter and thermo-gravimetric analysis. The inline melt flow properties were observed via vertical force measurements. The fracture morphology was investigated using SEM. Mechanical tests showed that the yield strengths of the composites were lower than that of pristine PP; in contrast, the elongation at break and the impact strength of composites enhanced in A-POSS and TS-POSS based composites.