Viscosity functions of particle filled polymer melts are shifted to higher values in comparison to those of the unfilled polymer melts. The extent of this viscosity increase is affected by a number of factors: the volume content, size and aspect ratio of the particles, interparticle interactions which includes the formation and breaking of agglomerates, the migration and rotation of particles during flow. The concept of shear-stress-equivalent inner shear rate according to W. Gleißle and M. K. Baloch describes the offset of the vertical shift of the viscosity functions of suspensions very vividly. Within this concept, the relative increase of the inner shear rate, which is defined as the shear rate of the continuous phase between the particles, is expressed by a shift factor. In this study, the flow behaviour of low density polyethylene (LDPE) filled with different volume fractions of wood particles and glass beads have been investigated by means of high pressure capillary rheometry in consideration on different particle sizes. The strongly pronounced shear thinning flow behaviour of the investigated polymer-compounds can be well described by the Ostwald – de Waele power law relationship. The measurements showed that interparticle interactions are differently pronounced depending on the size and aspect ratio of the particles. Interparticle interactions can be taken into account by the interaction exponent, defined as the ratio between flow exponent of the suspension and flow exponent of matrix polymer. The interaction exponent shows a good correlation with the flow consistency index. Consequently, this correlation function can be described as an interaction function. On the basis of a generalized interaction function, that considers the transition from negligibly interactions to the domain of pronounced interactions, the shift factor and thus the flow behaviour of the suspensions could be estimated with high accuracy.