In this study, ultra-high-molecular-weight polyethylene (UHMWPE)/gas-phase silica (GS) nanocomposites were prepared by the melt extrusion method. Scanning electron microscopy images of the cross-sectional morphology of the nanocomposites showed that the GS was dispersed uniformly in the UHMWPE matrix. Thermogravimetric analysis confirmed that the addition of GS resulted in an increase in the decomposition temperature of the UHMWPE matrix. X-ray diffraction showed that the GS had an obstructive effect on the crystallization of the UHMWPE. However, the thermal diffusivity of nanocomposites decreased with increasing the GS content, which confirmed that the thermal insulation properties of UHMWPE/GS nanocomposites were improved. Mechanical tests in tensile strength and elongation demonstrated that the strength of nanocomposites increased by the addition of an increased amount of GS.