The effectiveness of thermoelectric (TE) materials which can convert heat gradients into electricity and vice versa is quantified by the dimensionless figure of merit (ZT). Current TE materials such as Bi₂Te₃ and PbTe whose ZT values are around unity contain highly toxic and/or rare elements, limiting their widespread application. Silicon (Si) is a non-toxic, inexpensive, and earth-abundant element. Although bulk Si exhibits good electrical properties, its lattice thermal conductivity (κ_lat) is high (>100 W m⁻¹ K⁻¹), leading to the ZT value of around 0.01 at room temperature. If its κ_lat could be lowered while maintaining good electrical properties, Si could be an ideal TE material. These changes can be realized in Si by nanostructuring. Here we review recent results on the enhancement of TE efficiency of Si by nanoscale structure control. Based on the achievements of nanostructured Si in TEs, we point out some ideas for further enhancement of TE efficiency of Si.