The structural defects of linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) blends implanted with helium ions were investigated using positron annihilation lifetime (PAL) technique. The positron annihilation lifetime measurements were carried out at room temperature with a conventional fast-fast coincidence system. The lifetime spectra were analyzed into four components using the PATFIT program to extract the positron parameters such as lifetime components, and their corresponding intensities. Almost all parameters exhibited a correlation with microstructure changes resulting from implantation. The results were further discussed by comparison with modifications in the morphology of implanted samples using Scanning Electron Microscope (SEM) and Differential Scanning Calorimeter (DSC). Virgin Samples of Polyethylene blends are shown to be miscible by singlet DSC melting temperatures. After ion implantation the thermal properties of blends exhibit different behaviors depending on ion fluence and blending ratio i.e. polymer structure.