LDPE/montmorillonite and LDPE/kaolinite compounds were prepared by twin-screw extrusion using two screw profiles. The microstructure of the samples was determined by a combination of techniques: wide angle X-ray scattering (WAXS), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and small amplitude rheometry. This work presents also a new technique as a macroscopic measurement of the morphology of bulk filled polymers: in-line monitoring of low energy ultrasound waves during melt extrusion. Transit time, signal loss and peak height of ultrasound waves transmitted through the melt polymer were measured for all the samples. These parameters appeared to provide significant information of dispersion levels or aggregate size, and maintained a good correlation with the microscopic studies and the rheological data.