In this study, the effectiveness of three random copolymers based on blocks of poly(lactic acid) and polyethylene (PLA-PE) used as compatibilizer in PLA and high density polyethylene blends (PLA/HDPE) was evaluated in terms of thermal and mechanical properties. The compatibilizers were synthesized by different ways: reactive extrusion (C1), ring opening polymerization of lactide with PE with —OH substituent group (PE-OH) as initiator (C2) and polycondensation of lactic acid in presence of PE grafted with maleic anhydride (PE-g-MAH) (C3). The compatibilizers were used in two different proportions (5 and 15 wt%) and their efficiency was compared with a commercial compatibilizer (C0). The blends with C2 compatibilizer exhibited the highest tensile toughness (350% of increment compared with neat PLA/PE) because of the morphology attained. However, it was found that a relationship exists between the improvement of the compatibility and the loss of the thermal stability (−28°C) in PLA/HDPE blends. POLYM. ENG. SCI., 54:522–530, 2014. © 2013 Society of Plastics Engineers