Well-separated silicon nanowires with good periodicity and lower porosity density are fabricated using thermal oxidization and HF acid etching after forming silicon nanowire (Si NW) arrays with high diameter-to-pitch ratio by metal-assisted chemical etching based on nanosphere lithography. The factors responsible for the special morphology features of Si NW during the oxidation process are understood by thermal oxidation process simulation. A high-temperature oxidation process is proposed to facilitate the alleviation of necking and the acceleration of oxidation of NWs. Furthermore, to reduce the tapering trend, an oxygen diffuse barrier layer is proposed to be pre-deposited on the top of Si NWs before the high-temperature thermal oxidation treatment. By using these methods, a periodic array of Si NWs with a large pitch and small diameter is created. This approach can substantially reduce porosities and surface defects on the outer surface of Si NWs.