A simple method to fabricate circular microchannels in polydimethylsiloxane (PDMS) is presented. A coating of liquid PDMS is applied on the walls of rectangular microchannels, fabricated using standard soft-lithography, by introducing a pressurized air stream inside the PDMS filled microchannels. Surface tension of the liquid PDMS forces the coating to take a circular cross section which is preserved by baking the device to cure the coated layer. Diameters ranging from a few micrometers to a few hundreds of micrometers were achieved. The method was verified to work on microchannel networks as well as in straight channels. Different coating conditions were systematically tested. Design curves are reported for one to choose appropriate coating conditions for obtaining a desired diameter. A comparison between the performance of square and circular microchannels in trapping SiHa cells (cervical cancer cell line) is shown.