In order to realize the planar gradient refractive index (GRIN) microlens which is based upon porous silicon (PSi) and fabricated on silicon on insulator (SOI), a novel anodization method is used by applying lateral electric field. The microlens with smooth variation of the effective optical thickness is achieved. The lens is transparent in the infrared region, including the optical communication window (1.3 μm＜λ＜1.6 μm). This approach also allows the fabrication of an array of such lenses on SOI, and the GRIN microlens can be used as potential components in future silicon-based integrated optical circuits.