The control of the self-assembly of the nanocrystals into ordered structures has been extensively investigated, but fewer efforts have been devoted to studying one-component polymer-grafted nanoparticles (OPNPs). Herein, through coarse-grained molecular dynamics simulation, we design a novel nanoparticle (NP) grafted with polymer chains, focusing on its self-assembled structures. First, we examine the effects of length and density of grafted polymer chains by calculating the radial distribution function between NPs, as well as through direct visualization. We observe a monotonic change of the arranged morphology of grafted-NPs as a function of the density of grafted polymer chains, which indicates that the increase of the grafting density contributes to the order of the morphology. Meanwhile, we find that much longer grafted polymer chains worsen the regularity of the morphology. Then, we probe the influence of the stiffness of grafted polymer chains (denoted by K ranging from 0 to 500) on the order of grafted-NPs, finding that the order of the structure exhibits a nonmonotonic behavior as a function of K at moderate grafting density. For high grafting density, the order of the morphology is initially enhanced and becomes saturated as a function of K. For the effect of K on the stress-strain behavior, the system with the lowest order demonstrates the most remarkable reinforced mechanical behavior for both low and high grafting density. Last, we establish the phase diagram by varying the stiffness and density of the grafted polymer chains, which contains the amorphous, ordered, and superlattice structures, respectively. In general, our simulated results provide guidelines to tailor the self-assembly of the OPNPs by taking advantage of the length, density, and stiffness of grafted polymer chains.