Composites have been prepared from devulcanized waste ethylene propylene diene monomer (W-EPDM) rubber by blending with low-density polyethylene (LDPE) in a twin-screw extruder. W-EPDM was treated separately by optimized microwave and ultrasonic radiation. Effect of compatibilizer such as ethylene octane elastomeric copolymer (POE) on the enhancement of mechanical properties of composites was also studied in addition to the radiation treatment. The enhancement of tensile strength (TS) and elongation at break of composite with microwave-treated W-EPDM were increased up to 41% and 62%. Corresponding values were found 44% and 68% for ultrasonic treatment. Higher amount of energy during microwave treatment induced larger and localized cavity formation on the surface of composite results reduction of TS as confirmed from surface morphology study. On the contrary, encapsulation effect of POE only found to be useful to enhance TS of the composites up to 30% with untreated W-EPDM and LDPE. No significant additional strength increase over microwave and ultrasound treatment was found upon using POE. The encapsulation effect of POE disappeared when radiation was applied. FTIR and DMA study revealed the existence of only physical interaction upon microwave and ultrasound treatment. POLYM. ENG. SCI., 55:533–540, 2015. © 2014 Society of Plastics Engineers