In this work, the first highly controllable assembly and rotation of silicon nanowires and nanomotors in suspension are reported. Si and Si composite nanowires are fabricated with precisely controlled dimensions via colloidal assisted catalytic etching. The nanowires can be rotated with deterministic speed and chirality. The rotation speed and orientation not only depend on the applied AC electric frequency, but also on the electronic type, geometry, surface coating, as well as the electric conductance of suspension mediums. Theoretical analysis is used to understand the rotation of Si nanowires, and also the electric resistivity of Si nanowires is determined from their mechanical rotation. The Si nanowires are precisely assembled into nanomotors that can be rotated with controlled speeds and orientations at prescribed locations. This work provides a new paradigm for designing and actuating various Si-based nanoelectromechanical system (NEMS) devices, which are relevant to man-made nanomotors, nanorobots, and nanoengines.