This Letter presents a novel fabrication method to build polymer microlens arrays. The key mechanism of the fabrication process is surface tension, which induces liquid separation phenomenon when a post is immersed and detached from a bottom liquid mixture. The surface tension also acts on the formation of lens surface at liquid-air interface resulting in an excellent surface roughness. The surface profile of lenses is well fitted with an ideal lens profile. The other feature of the presented fabrication method is self-positioning of the lens into the post. The range of focal lengths was between 110.4 and 282.0--m, which were determined by three parameters such as a post radius, gap and bottom liquid thickness. In summary, the STILS method can be a powerful tool to create microlens arrays with a smooth surface and a high scalability for the future applications in optoelectronics or microfluidics.