It is well known that adding particular particles to thermoplastic matrices can greatly improve the properties of the resulting composite. Graphene is one such particle as it has remarkable properties such as excellent rigidity, good electrical and thermal conductivity and even barrier properties against water and oxygen. However, the desired improvement depends greatly on the morphology of the thermoplastic/graphene composite. One way to control the morphology is by using adequate processing techniques and parameters. Previous studies have shown that applying ultrasound during melt extrusion can impact the dispersion of nanoparticles inside thermoplastics, however, this phenomenon is not yet completely understood nor controlled. In this work, the effect of applying ultrasonic vibration during melt extrusion on the dispersion of graphene nanoplatelets (GnP) inside high-density polyethylene (HDPE) was investigated. HDPE masterbatches with 17 wt% of GnP were processed using three different amplitudes of ultrasound. Then, the masterbatches were diluted via melt extrusion to obtain HDPE nanocomposites with 0 to 17 wt% GnP. The dispersion of GnP was assessed using small angle oscillatory shear tests and electrical conductivity tests. Visual observations of the composites were also done using scanning electron microscopy. It was shown that the dispersion state of GnP in HDPE was dependent on the amplitude of ultrasound applied during melt extrusion.