Study of the decomposition kinetics, pyrolysis products, and even reaction mechanisms plays an important role for the development of polymer recycling. In the present research, the kinetics of virgin and waste polypropylene (PP) and low-density polyethylene (LDPE) were studied by a modified Coats-Redfern method. Afterward, thermal cracking of them in a semibatch reactor under atmospheric pressure in nitrogen has been investigated. Both virgin and waste plastics are decomposed at 420-460 degrees C, and the products have been characterized using GC, FT-IR, H-1 NMR, and GC-MS. The reaction path and the degradation mechanism for the thermal cracking of polymer in this study were also discussed. The lower activation energy of waste PP and LDPE indicates that waste plastics degrade at lower initial temperature and may favor mild conditions. Due to the short residence time, the higher gaseous and liquid yields were obtained for virgin PP and LDPE. A large amount of residues for waste polymer indicates that it is a favorable way to degrade waste plastics in a semibatch reactor without further separation. Chain scission reactions are the predominant degradation mechanism in this thermal cracking process. The significant content of unsaturated hydrocarbons in PP thermal cracking products shows the intramolecular hydrogen transfer and beta-scission reactions are predominant. In the case of LDPE, intermolecular hydrogen transfer and beta-disproportionate reactions also occur. For thermal cracking polymer in a semibatch reactor under atmospheric pressure, the high yields of gasoline (C-6-C-12) and diesel (C-13-C-22) fraction in liquid products confirm that it is a desirable way to realize waste plastics recovery.