Physico-chemical properties of polypropylene (PP) composites can be tailored through the use of organoclay fillers in the presence or absence of compatibilizer. In this study, polypropylene grafted p-hydroxy-N-phenyl maleimide (PP-g-pHPMA) was used as compatibilizer and organo-modified montmorillonite (OMMT) that was prepared by the modification of MMT with hexadecyl trimethyl ammonium bromide surfactant (OMMT) was used as filler. PP/OMMT composites with different percentages of OMMT loading (1-7 wt%) both in absence and presence of PP-g-pHPMA (3.0 wt%) were prepared by melt blending. The effect of organoclay content and compatibilizer on the physical properties of the prepared nanocomposites have been studied using melt flow index measurements (MFI), X-ray diffraction analysis (XRD), thermogravemetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and both oxygen and water vapor transmission rates. Both XRD and transmission electron microscopy (TEM) results suggested the intercalated/exfoliated structure of the prepared nanocomposites. MFI results showed a great increase in the value of the MFI by increasing OMMT content and the increase is lower in the presence of compatibilizer. TGA results indicated that the thermal stability of PP was enhanced with the incorporation of both OMMT and PP-g-pHPMA. Melting and crystallization behavior of the prepared PP nanocomposites was slightly changed. Water vapor and oxygen barrier properties of the prepared composites were found to be improved by increasing OMMT content.