Much attention has been paid in recent years to the toughening of isotactic polypropylene (PP)，and blending PP with ethylene-propylene terpolymer (EPT) is one of the most effective ways．In order to further improve the toughness of PP／EPT blend and to enhance the stabi-lity of the dispersed rubber phase，a diblock copolymer composed of PP and ethylene-propylene rubber blocks (PP-EPR) was added into the blend．as it is considered that“forced compatibi-lity” of the two components will be effected through the two chemically bonded blocks，while two-phase structure still remains．Experimental results indicate that PP-EPR is an effective compatibilizer for PP／EPT blend．It not only increased the toughness of the blend，but also decreased the loss of tensile strengths usually accompanying the toughening of plastics with an elastomer，which is very well explained by the changes in morphological structure when an appro-priate amount of PP-EPR was added into the blend．