The polymeric nanocomposites have attracted attention in recent years because of impressive enhancements of material properties as compared with the pure polymers. Intrest in polyolefin-nanocomposites has appeared due to their improved performance in engineering applications. Among the different nanoparticles, nanoclay and Calcium carbonate (CaCO3) are commonly used nanofillers. Recently co-incroperation of two nanofiller have attracted attentions. The aim of this work is to evaluate the simultaneous effect of two different types of nanosized fillers (nanoclay and nano CaCO3) with different aspect ratios on the rheological properties and morphology of HDPE/nano clay/nano CaCO3 nanocomposites. Ternary Nanocomposites of HDPE/nanoclay/nano CaCO3 were prepared using melt mixing. Maleic anhydride grafted polyethylene (MA-g-PE) was used to enhance the dispersion of nanofillers and better interface adhesion. One level of nanoclay ( 3 wt. %) and Three different levels of CaCO3 (6, 8, 10 wt. %) and MA-g-PE (3, 6, 9 wt. %) were used. The mixing was done in two steps: First a concentrated masterbatch of nanoparticles in HPDE and MA-g-PE was prepared using an internal mixer and then melt-mixing of nanocomposites was done in a lab scale co-rotating twin screw extruder. The microstructure of samples was studied through different techniques. According to the SEM micrographs, nanofillers were well dispersed in the HDPE matrix. XRD patterns showed the intercalation of nanoclay layers in presence of nano CaCO3 and MA-g-PE. Rheological studies demonstrate that by increasing of CaCO3 and MA-g-PE content, a rise in complex viscosity, elastic modulus and decrease in elastic modulus slope can be observed in terminal region. All of these techniques, showed intercalation of nanoclay and good dispersion of both nanoparticles.