Molecular dynamics simulations of three kinds of linear low-density polyethylene (LLDPE) single-chain models with precisely controlled branching are performed. It is shown that the crystallinity of copolymers with branches shorter than C10H21 decreases with increasing branch length, whereas for copolymers having branches longer than C10H21, the crystallinity increases as the branch length increases. From the simulations of ethylene/vinyl chloride copolymer model, it is found that the crystallization process and driving force of LLDPE chain with polar comonomer are similar to those having a nonpolar comonomer, and the MD simulations of the models with branches of different flexibilities show that as branch flexibility decreases, the side-chain co-crystallization becomes more difficult, and the corresponding lamella is packed more loosely.