The present work reports the empirical findings of the effects of mean particle size and weight percentage of CaCO3 on selected rheological characteristics of filled LDPE. The experimental results indicated that the two parameters studied had noticeable influence on the selected rheological properties of LDPE. Studies on the slip velocity and critical shear stress based on Mooney analysis revealed that the designed benefits brought by the incorporation of additives (such as CaCO3) into a polymer (such as LDPE) may not be justified by their possible adverse effects. It is also shown in this report that melt index and apparent viscosity of the selected polymer systems were well correlated with mean particle size of CaCO3 and its weight percentage.