Thermal decomposition behaviors of neat polypropylene (PP) and low-density polyethylene (LDPE) and LDPE/PP mixture with different ratios (30/70, 50/50, 70/30) are studied under nonisothermal conditions in closed-batch reactors. The presence of a synergistic effect is observed for mixture samples with a PP composition of >= 30 wt %. Different proportions of PP in LDPE/PP mixtures give different interaction effects. The 70/30 LDPE/PP mixture starts to give viscosity-reducing effect and can degrade at lower temperature; the 50/50 LDPE/PP mixture gives higher gaseous products yield, specifically, more methane, ethane and propane; and the 30/70 LDPE/PP mixture makes more aromatics formed in the liquid products. The synergetic effect of LDPE and PP is related not only to the mass fraction of PP, but also to the reaction time. The interactions mechanism are related to enhanced intermolecular hydrogen transfer between LDPE/its cracked products and PP/its cracked products, not only in the liquid phase but also in the gas phase, because of the long cracking time in closed-batch reactors, where PP/its cracked products provide more radicals and PE/its cracked products provide more secondary hydrogen for intermolecular hydrogen transfer, and the combination of these factors enhances the chain breakage. This study also indicates that LDPE/PP mixture increases the aromatics formation mainly by increasing olefinic content due to the more beta-scission reactions in the LDPE/PP mixture mild cracking.