We present the fabrication and thermal conductivity of a high-density and ordered 10 nm-diameter Si nanowires (SiNWs) array for thermoelectric devices, realized through the use of a bio-template mask as well as neutral beam etching techniques. The SiNWs were embedded into spin-on-glass (SoG) to measure the thermal conductivity of the SiNWs-SoG composites. By decreasing the thickness of SiNWs-SoG composites from 100 nm to 30 nm, the thermal conductivity was drastically decreased from 1.8 ± 0.3 W m−1 K−1 to 0.5 ± 0.1 W m−1 K−1. Moreover, when the electrical conductivities of 100 nm-long SiNWs were 1.7 × 10 S m−1, 6.5 × 103 S m−1 and 1.3 × 105S m−1, their thermal conductivities of SiNWs-SoG composites were 1.8 ± 0.3 W m−1 K−1, 1.6 ± 0.2 W m−1 K−1 and 0.7 ± 0.2 W m−1 K−1, respectively. The cross-plane thermal conductivity of the fabricated 10 nm diameter SiNWs-SoG composites was dependent on their thickness and the electrical conductivity of SiNWs, which were significantly decreased from bulk.