This report describes the use of patterned paper as a low-cost, flexible substrate for rapidly prototyping PDMS microdevices via "liquid molding". The entire fabrication process consists simply of three steps: (1) fabrication of patterned paper in NC membrane by direct wax printing (or modified wax printing that we call "transfer wax printing"); (2) formation of liquid mold on wax-patterned NC membrane; (3) PDMS molding and curing on wax-patterned NC membrane anchored with liquid micropatterns. All these procedures can be finished within only 1.5 h without the use of a photomask, photoresist, UV lamp, etc. Through the use of wax-patterned NC membrane coupled with a liquid mold as a template, different PDMS microdevices such as microwells and microchannels have been fabricated to demonstrate the usefulness of the method for PDMS microfabrication. The height of microwells and microchannels can also be tailored flexibly by adjusting the liquid filling volume. This method for prototyping PDMS microdevices has some favorable merits including simple operation procedures, fast concept-to-device time, and low cost, indicating its potential for simple PDMS microdevice fabrication and applications.