This work reports, for the first time, on the self-assembly of polystyrene (PS) droplets into a uniform layer at a PVDF/HDPE interface in the case of that intermediate phase (PS phase) as low as 6% during melt processing. Firstly, PVDF-g-PS has been successfully synthesized utilizing electron beam induced free radical graft copolymerization technique and copolymer was characterized by FTIR, DSC and 1H NMR. For the unmodified PVDF/PS/HDPE (52/6/42 vol%) ternary blend, the morphology was that a portion of the PS droplets arrayed at the PVDF/HDPE interface, another fraction was located inside the HDPE phase. More and more PS migrated to the interface of PVDF/HDPE at the addition of PVDF-g-PS, a perfect tri-continuous morphology that PS droplets formed a uniform layer arrayed at PVDF/HDPE co-continuous interface was achieved when the weight fraction of PVDF-g-PS was 10%.