In this work mechanical recycling of waste poly(ethylene-terephthalate) (PET) in the presence of high density polyethylene (HDPE) was studied. The fraction of HDPE in PET/HDPE blends was 3, 5, 6 and 10 wt.%. Isocyanate HI compatibilizer was first synthesized and then added to the blends at a fraction of 5 wt.%. Ethylene-propylene-diene masterbatch (EPDM-M) was prepared by mixing and added in fraction of 15, 30 and 50 wt.%. All PET/HDPE blends studied were characterized by scanning electron microscopy SEM, differential scanning calorimetry DSC and melt flow rate (MFR). Blends prepared with isocyanate HI compatibilizer were also studied by FTIR spectroscopy. Appearance of new vibrations in the FTIR spectra confirms that during the blending reactions between isocyanate HI compatibilizer and blend components appear Results show that used compatibilizers significantly improved compatibility of polymers in studied blends. From the results it is obvious that immiscibility of PET and HDPE can be overcome.