Influences of butadiene rubber (BR) on the structure and mechanical properties of poly(vinyl chloride) (PVC)/low density polyethylene (LDPE) blends and its synergism with crosslinking agent have been studied. As a solid phase dispersant, BR can promote the phase dispersion of PVC and LDPE, but cannot improve their interfacial adhesion. While dicumyl peroxide (DCP) can initiate the co-crosslinking of PVC and LDPE and hence improve their interfacial adhesion, its efficiency is limited because of the poor phase dispersion of the blend. When BR and DCP are added to the blend simultaneously, the probability of DCP existing at the interface will increase and more co-crosslinking products will form. Therefore, good synergism will be caused and the mechanical properties will be modified due to the improvement of the interfacial adhesion.