High-density polyethylene–low-density polyethylene (HDPE-LDPE) blends involving HDPE-LDPE (50/50 wt%) with dialkyl peroxide (DAP; 0.05 wt%) containing different amounts (5, 10 and 15 wt%) of nanocalcium carbonate (nCC) were prepared by melt blending followed by compression molding. The effect of addition of nCC on the morphology, mechanical and crystallization behaviors of HDPE-LDPE blends with and without DAP was also evaluated. The measurements of the mechanical properties showed that the impact strength of the nanocomposites increased at first and then decreased with the addition of nanofillers, and the tensile strength was reduced at the same time. With the addition of a small amount of DAP, the mechanical properties of the obtained nanocomposites increased. Scanning electron microscopy observation showed that the addition of DAP improved the dispersion of the nanofiller in the HDPE-LDPE blends. Differential scanning calorimetric results indicated that the addition of DAP increased the crystallization temperature as a result of heterogeneous nucleation effect of nCC on HDPE-LDPE blends.