Polymer blends have the potential to be developed into new resins for specific applications with balanced performance properties and economic advantages. In this paper, polypropylene (PP) and linear low-density polyethylene (LLDPE) blends were investigated at specific composition of PP/LLDPE (60/40) blends (PL40) using a Twin Screw Extruder (TSE). The nano-calcium carbonate (CaCO3) content was varied (between 0-40 wt%), and the composites were investigated using slit die rheometers. Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) techniques, revealed the existence of favorable interaction between the surface of CaCO3 nanoparticles and PL40 matrices after surface modification of aminopropyltriethoxy silane (AMPTES) coupling agent. Rheological properties reveal that both the apparent viscosity and the pseudo-plasticity of the PL40 nanocomposites increases with increasing nano-CaCO3 contents and decreases slightly by treated nano-CaCO3 with 2% AMPTES. Dynamic mechanical analysis (DMA) demonstrated that the storage modulus (G′) and glass transition temperature (Tg) are enhanced upon treated nano-CaCO3 with a coupling agent. The results from the mechanical properties exhibited that the impact strength was optimum at 20% nano-CaCO3 content, but tensile strength slightly decreased for untreated PL40 nanocomposites. However, treatment with amino coupling agent improved tensile strength. This attribute to the enhancement of dispersion of the nano-CaCO3 particles in PL40 matrix, leading to improved interfacial adhesion. Treatment of nano-CaCO3 significantly improved the Young’s modulus and flexural modulus compared to the untreated PL40 composites. The crystallization behavior of the composites supported the strong interfaces formed on addition of the amino coupling agent. Furthermore, scanning electron microscopic (SEM) showed evidence of improvement in the dispersion of treated CaCO3 in the PL-40 matrix.