The flammable and the mechanical effects of silica (SiO₂) on the halogen-free, flame-retarded ethylene–octene copolymer (POE)/polypropylene (PP) composites were investigated. Through the thermal stability analysis and the combustion test, it was found that the SiO₂ had a synergistic effect on the intumescent flame retardant (IFR), especially when the SiO₂ content was 3 wt%. According to the results of mechanical properties, the tensile strength of composites increased with the rising SiO₂ content and most of the impact strength is retained. The fracture morphology showed the state of SiO₂/IFR particles in the POE/PP blends, which reflected that with the increasing SiO₂ content and the decreasing IFR content, the aggregates became less compact and the size of aggregates became smaller.