In this work an innovative microelectrode was successfully fabricated using photolithography for determination of dissolved oxygen distributions in aerobic granules, which were sampled from a nitrifying sequencing batch reactor. A negative photoresist, SU-8, was used as a substrate for the microelectrode and a 70 microm wide needle was photopatterned on it. The microelectrode could be renewed several times. Cyclic voltammetry analysis and dissolved oxygen measurement demonstrated that the microelectrode was stable and reliable. Dissolved oxygen distribution in a nitrifying granule was successfully monitored with the microelectrode. The profiles indicated that the main active part of the nitrifying granule was the upper 150 microm layer. Using the procedures developed in this work, microelectrodes of the desired shape could be constructed with precise size control at micrometers-scale.