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The effects of graphene nanoplatelets (GNPs) on the nucleation of the &#946;-polymorph of polypropylene (PP) were studied when melt-mixed at loadings of 0.1&#8211;5 wt % using a laboratory scale twin-screw (conical) extruder and a twin-screw (parallel) extruder with <i>L/D</i> = 40. At low GNP loadings (i.e., &#8804;0.3 wt %), the mixing efficiency of the extruder used correlated with the &#946;-nucleating activity of GNPs for PP. GNP agglomeration at low loadings (&lt;0.5 wt %) resulted in an increase in the &#946;-phase fraction (<i>K_&#946;</i>) of PP, as determined from X-ray diffraction measurements, up to 37% at 0.1 wt % GNPs for composites prepared using a laboratory scale twin-screw (conical) extruder. The level of GNP dispersion and distribution was better when the composites were prepared using a 16-mm twin-screw (parallel) extruder, giving a <i>K_&#946;</i> increase of 24% upon addition of 0.1 wt % GNPs to PP. For GNP loadings &gt;0.5 wt %, the level of GNP dispersion in PP did not influence the growth of &#946;-crystals, where <i>K_&#946;</i> reached a value of 24%, regardless of the type of extruder used. From differential scanning calorimetry (DSC) measurements, the addition of GNPs to PP increased the crystallization temperature (<i>T_c</i>) of PP by 14 &#176;C and 10 &#176;C for the laboratory scale extruder and 16-mm extruder, respectively, confirming the nucleation of PP by GNPs. The degree of crystallinity (<i>X_c</i>%) of PP increased slightly at low GNP additions (&#8804;0.3 wt %), but then decreased with increasing GNP content.