Polymer-based nano/microfluidic devices are becoming increasingly important for biological applications and fluidic control. Reported is a new etching method for the fabrication of nano/microfluidic channels based on SU-8 using AZ1350 as a sacrificial resist. In contrast to all the previous fabrication routes, this etching method is suitable for fabricating the channel with dimensions ranging from micrometres to nanometres. By this route, the most critical step is to prevent the two mixed photoresists and this problem is solved by sputtering a thin layer of SiO2. Furthermore, this is a size-controlled nanochannel fabrication method because the size of the channel is only dependent on the sacrificial layer structure whose size could be controlled by the oxygen plasma process. In addition, the developing etching speed is measured and some methods to accelerate the developing etching rate are proposed. This novel process is simple and inexpensive for mass nano/microchannel manufacturing, which could have wide applications in biomedical and fluidic transport systems.