We report a new patterning method for single-walled carbon nanotubes (SWCNTs) films on flexible, transparent polyethylene terephthalate) using electrochemical etching in an aqueous electrolyte solution. Electrochemical etching of the SWCNT films patterned with photoresist polymer was accomplished in a three-electrode system, and the electrochemically patterned SWCNT films were then characterized by scanning electron microscopy (SEM) and Raman spectroscopy. The voltammetry curve showed that SWCNTs underwent drastic oxidation above an applied potential of 1.315 V with the generation of gas bubbles, and the oxidation current became constant above 2.6 V due to the mass transfer limit. SEM images showed that the networks of SWCNTs in the area protected with the photoresist polymer had no damage and vivid connections were obvious, while the connections and shapes of SWCNTs in the area exposed to electrochemical etching were indistinct and slightly damaged. In the Raman spectra of the area protected with the photoresist polymer and the exposed SWCNT area, the intensity ratio of the D-line to the G-line increased from 0.077 to 1.136, which indicated that the ordered carbons of the SWCNT film gradually became amorphous carbons due to electrochemical etching. For optimal patterning, the electrochemical etchings of SWCNT films were performed under various conditions (the applied potential, pH of the electrolyte solution, and electrolyte concentration). An applied potential of 3.0 V in 0.1 M NaCl electrolyte solution (pH 7.0) was optimal for homogeneous electrochemical patterning of SWCNT films. In an electrochemiluminescence reaction, the SWCNT films patterned by this technique could be used successfully as flexible and transparent electrodes.