Correlation between structure and mechanical properties of linear-low density polyethylenes (LLDPE) was studied with LLDPEs of different crystalline structure. From the viewpoint of crystalline structure, the tested polymers were classified into two types; one with shorter long periods with no phase-separated domains and the other with longer long periods with phase-separated domains. From the low-temperature impact test results, crystalline structure, more specifically the number density of tie molecules, appeared to be a more critical factor determining impact resistance of semicrystalline polymers than the presence of phase-separated domains is.