A series of N-octadecylated polyethyleneimine (PEI18C) comblike polymers with different grafting ratios, through N-alkylation method, have been successfully prepared. The chemical structure and thermal behavior of PEI18C comblike polymers with different grafting ratios has been characterized by Fourier transformed infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Experimental shows that C18 alkyl groups have been successfully grafted onto PEI backbones, and the thermal behavior of PEI18C comblike polymers with different grafting ratios exhibit the linear increment with the grafting ratio of alkyl groups. It can be attributed to the effect of the grafting density of alkyl side groups along PEI backbones. The detailed investigation and discussion is carried out from the viewpoint of side chain crystallization and molecular packing behaviors.