A multistep processing method was developed to fabricate a partial wetting morphology for improving toughness of flame retardant polymer. In the first step, high‐density polyethylene (HDPE) and nylon 6 (PA6) were melt‐extruded with intumescent flame retardant (IFR) for fabricating HDPE/PA6/IFR blends with a core–shell structure (core: IFR, shell: PA6). At the second step, maleic anhydride‐grafted‐linear low‐density polyethylene (LLDPE‐g‐MAH) was melted with HDPE/PA6/IFR at processing temperatures slightly below the melting temperature of PA6 to produce a partial wetting morphology in which LLDPE‐g‐MAH phase with the sphere was dispersed at the HDPE/PA6 interface. The effect of the LLDPE‐g‐MAH content on the impact strength was investigated, and high toughness was exhibited in the blend with 2 wt % LLDPE‐g‐MAH. Its elongation at break and notched impact strength were 43 and 270% higher, respectively, than that of neat HDPE. The unique interface failure mode was responsible for the high impact strength. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48735.