A simple method to release Si nanowires (SiNWs) from a substrate, with their original length almost intact, is demonstrated. By exploiting the unique chemistry involved for the fabrication of vertical arrays of SiNWs in metal-assisted chemical etching (MaCE) based either on HF/AgNO3 or HF/H2O2 chemistries, wet etching with alkali hydroxides such as NaOH or KOH preferentially attacks the bottom part of the vertical SiNWs. A protective layer of Si oxide is found to exist on the outer wall of the SiNWs and to play the key role of etch mask during the release-etching by alkali hydroxides. The clean release of SiNWs also enables the repeated use of the Si substrate for the fabrication of vertical SiNW arrays by MaCE. The released SiNWs are further used for the fabrication of field-effect transistors on a flexible plastic substrate. The method developed here, when combined with a suitable assembling technique, can be very useful in implementing flexible electronics, or in the fabrication of SiNW composites with other functional materials.