The phase and mechanical properties of a low-density polyethylene/polystyrene 70/30 (LDPE/PS 70/30) blend has been determined after compatibilization with various styrene-ethylene, styrene-ethylene/butene and styrene-ethylene/propylene type block copolymers. It is shown that high interfacial activity, as reflected in the reduction of the dispersed phase size, does not necessarily bring about an improvement in the mechanical properties of the blend. Although the diblock copolymers were more efficient in reducing the phase size, the triblock copolymers were more effective in improving the mechanical properties. The effect of a poly(styrene-b-ethylene) (S-E) block copolymer on the particle coalescence and rheological properties of the LDPE/PS 70/30 blend has been investigated. It is shown that small amounts of the block copolymer can significantly retard or actually suppress the coalescence of the dispersed phases, and thus stabilize the morphology of the blend. The addition of the S-E copolymer increases the elastic modulus and dynamic viscosity of the blend, and the samples become more non-Newtonian as the block copolymer concentration increases.