Natural rubber (NR)/low density polyethylene (LDPE) composites are utilised to manufacture rubber-based articles. Properties of these composites become inferior if they are used alone due to the incompatibility of the base polymers. Peroxides help to cross-link both NR and LDPE, and the addition of peroxide to the composite at its optimum loading may enhance properties by developing a stable phase morphology. Therefore, the present work investigates the effect of dicumyl peroxide (DCP) loading on physicomechanical properties of calcium carbonate (CaCO3)-filled NR/LDPE (70/30) composites. The composite preparation was based on varying the DCP loading from 0 to 0.9 parts per hundred parts of polymer (phpp) at 0.1 phpp intervals. The composite at DCP loading of 0.3 phpp indicated the highest physicomechanical properties, gel content, the hardness of gel, and degree of crystallinity and the lowest degree of swelling. Properties of the composites with DCP were better than those of the control (without DCP). Water absorption decreased with the addition of DCP up to 0.3 phpp and increased after that with the rise in DCP loading. Scanning electron microscopic (SEM) image of the composite at DCP loading of 0.3 phpp revealed a smooth fracture surface. Therefore, 0.3 phpp was identified as the optimum DCP loading for the NR/LDPE (70/30) composite.