The microstructure of carbon-black (CB) filled high density polyethylene/Ethylene Propylene Diene Monomer (HDPE/EPDM) composite was studied with positron annihilation lifetime spectroscopy (PALS) and positron annihilation Doppler spectroscopy (PADS). It was found that tau(3) and I(3) of the long lifetime component of the PALS spectra show non-linear relations with the EPDM content. It was also noticed the DBS was very sensitive to the microstructural change in the HDPE(CB)/EPDM composites. The spatial distributions of the EPDM and the CB in the polymer are also discussed.