CBMB (carbon black masterbatches) contain up to 40 wt.% carbon black (CB). Good dispersion of CB in polyolefins is often difficult to achieve affecting the processability and properties of the materials. In practice, a low molecular weight (MW) unimodal polyethylene (PE) is employed as a carrier for CBMB. Since CBMB are used for protecting high density polyethylene (HDPE) pressure pipes against photo-degradation, it is important that the CBMB carrier resin used in plastic pipe applications has a high MW in order to maintain superior mechanical properties [1], while keeping the same flow behaviour as CBMB based on à low MW carrier resin. In this work, the dispersion of CB in bimodal polyethylene was investigated. Due to its low viscosity, the low MW fraction could penetrate into the CB surface pores improving the wetting, which also lead to enhanced mechanical interlocking at the polymer/filler interface and efficient stress transfer. The high MW fraction on the other hand exerts high enough shear stress to break up the filler agglomerates during compounding. Figure 1 illustrates the effect of polymer design on CB dispersion (resin MFR and reactor % split). Sample a represents the CB dispersion in the reference unimodal HDPE carrier having MFR2=11 g/10 min. Not dispersed and distributed CB agglomerates are clearly visible on the micrograph. On the other hand, the CB is well dispersed in sample b having MFR2=13 g/10 min. A possible reason could be the sufficient amount of low MW loop reactor fraction to wet and infiltrate the filler particles. The effectiveness of CB dispersion can be controlled by having low enough viscosity for wetting/infiltration the filler and a high MW fraction to attain a shear stress above the critical value to break-up the filler agglomerates. It was also found that CB dispersion can be correlated to the loss tangent data (tan delta) from rheological measurements.