With the development of 5G technology, the miniaturised and highly integrated electronic devices urgently require thermal management materials possessing high thermal conductivity and mechanical properties. In this work, isotactic polypropylene (iPP)/high-density polyethylene (HDPE)-based dielectric composites possessing ideal thermal conductivity and balanced mechanical properties were prepared via Fused Filament Fabrication (FFF). The advanced material properties were achieved by the introduction of hybrid fillers and tailored polymer crystalline structure. The highly oriented h-BN, oriented iPP crystalline and iPP/HDPE epitaxy crystalline were observed. Meanwhile, we studied the effect of the ratio of hybrid fillers on various properties of composites. The thermal conductivity of iPP/HDPE/h-BN/Al2O3 composites reach 1.802 W·m−1·K−1. The impact strength and tensile strength reach 13.23 KJ/m2 and 40 MPa, respectively. In addition, the composites maintain ideal dielectric properties. This work offers a feasible strategy to fabricate dielectric and thermal conductive composites with balanced mechanical properties using semicrystalline polymer through FFF process.