The effect of processing conditions, nucleating agent and compatibilizer on the crystallinity, mechanical property and oxygen permeability of polypropylene was studied using differential scanning calorimetry, x-ray diffraction, thermogravimetric analysis and universal tensile testing. The as received polymer had much lower extent of crystallinity, which was enhanced to 58% by the processing and molding using either slow cooling or fast cooling conditions. Although the overall crystallinity was not affected by the rate of cooling, the crystallization behavior was, however, affected as indicated by different onset and peak crystallization temperatures as well as different crystallite sizes indicated by the differences in oxygen permeation and mechanical performance of the polymer. Nucleating agent further enhanced the extent of crystallinity (crystal plane peaks were shifted and broadened) and tensile modulus, but the polymer was observed to become brittle and had higher gas permeation through it. The compatibilizer, on the other hand, reduced the tensile modulus, but enhanced the yield strain owing to matrix plasticization and retained the similar oxygen permeation as the polymer. The temperature used during processing also affected the oxygen permeation and mechanical property as the high temperature may induce partial polymer degradation resulting in the reduction of molecular weight and deteriorated properties.