This paper describes the synthesis of hydrogenated polybutadiene-b-polyamide 6 (HPB-b-PA6), a pure diblock copolymer, which consists of three main steps: synthesis of hydroxyl terminated polybutadiene, its hydrogenation and functionalization with an epsilon-caprolactam blocked diisocyanate. In a third step the functionalized HPB is copolymerized with epsilon-caprolactam monomer via ring opening anionic polymerization. The compatibilization ability of the synthesized pure diblock copolymer was evaluated in low-density polyethylene (LDPE)/polyamide 6 (PA6) blends. The investigation includes phase morphology development using both optical and electron scanning microscopy. The ultimate tensile properties of the modified and non-modified blends were also evaluated. The synthesized pure diblock copolymer exhibits very interesting interfacial activity both in terms of particle size reduction and improvement of the interfacial adhesion between the incompatible LDPE/PA6 phases. The tensile properties of the investigated blends were also significantly affected by the addition of the copolymer. The efficiency of compatibilization was found to be very sensitive to the molecular characteristics such as composition and molecular weight of the copolymer. Among the copolymers investigated a copolymer containing 24 wt% PA6 and having a total molecular weight (M) over bar (a) of 87,000 was found to exhibit the most efficient interfacial activity.