Linear low-density polyethylene and polypropylene (LLDPE/PP) blends were compatibilized with peroxide using two distinct blending techniques and the blend properties were compared. One was the concurrent blending method in a polymerization reactor (reactor blend), and the other was the conventional melt blending in a twin-screw extruder (extruder blend). Scanning electron microscopy revealed that the reactor blend resulted in dispersed PP particles less than 1 mu m in diameter, whereas the particles were 3 to 4 mu m in diameter for the extruder blend. The rheological properties judged from Rheometrics mechanical spectrometry, capillary rheometry, melt index, complex viscosity, and melt strength also suggested different natures for the two types of blends. The extruder blend showed a sharp change in flow properties as a function of peroxide and coagent concentration, whereas this was only gradual for the reactor blends. For mechanical properties, the reactor blends were superior in elongation at break to the extruder blends. These observations suggest that the level of interfacial tension was lower for the reactor blends than for extruder blends. This could be interpreted as taking LLDPE/PP copolymer formation at the interface into account.