The underfill process is a key electronic packaging technology for chips based on integrated circuits (ICs).[1] It is a major bottleneck to the design and fabrication of epoxy -based electronic packaging materials with excellent thermal conductivity (≥1 w/mK), high electrical insulation and low viscosity (<20 kcps at 25 oC).[2] Although carbon nano -tubes (CNTs) have superior thermal conductivity, when added to polymer matrices, the increase in thermal conductivity of the composites is very much less than that predicted owing to the high thermal interface resistance.[3] Recently, we synthesized silica-coated multi-walled carbon nanotubes (MWCNT@SiO2), and found that the silica shell on the CNTs helped retain the high electrical resistivity of the epoxy matrix. Also, the modulus mismatch between the stiff CNTs and soft epoxy matrix reduced the thermal interface resistance, yielding a 67% increase in the thermal conductivity of an epoxy/MWCNT@ SiO2 composite containing only 0.52 vol.% fillers.[4] However, this thermal conductivity is still many times lower than 1 w/m∙K. Silver nanowires (AgNWs), as one-dimensional nanostructured materials, possess high aspect ratio and intrinsically high thermal conductivity. However, AgNWs are difficult to disperse homogeneously in epoxy, and their high electrical conductivity also limits their application for electronic packaging. In this new study, silica-coated sliver nanowires (AgNWs@SiO2) were synthesized by a sol-gel method and then incorporated into epoxy. Results showed that the thermal conductivity of an epoxy/AgNWs@SiO2 composite with 4 vol.% fillers was increased to 1.03 W/m∙K from 0.19 W/m∙K of neat epoxy (4-fold increase), while the thermal conductivity of an epoxy/AgNWs composite with the same filler loading was 0.57 W/m∙K. Simultaneously, the insulating silica nano -layer effectively avoided the formation of electrically conductive networks of AgNWs in the epoxy matrix, leading to the high electrical insulation of the composite.