The study focused on the discharge mechanism of plasma electrolytic oxidation (PEO). Titanium with a layer of aluminum deposited on it (Ti-Al) was used as the substrate. When PEO process for Ti-Al substrate developed for a period of time, by analyzing the distribution of Ti element in oxide coatings, the existence of oxide film dielectric breakdown in discharge process was testified and the general shapes of discharge channels were also obtained. From the study on discharge channels' shapes we found that: in most of discharge locations, the coatings were broken down partly, while there also existed some discharge places in which the coatings were broken down thoroughly. Moreover, it was found that the oxide films grew toward the outer as well as the interior of PEO coatings, through analyzing the distributions of Si and Ti elements in discharge channels. [DOI:10.1016/j.surfcoat.2012.05.002] The article was published as: Liu XJ;Li G;Xia Y. Investigation of the discharge mechanism of plasma electrolytic oxidation using Ti tracer. SURFACE & COATINGS TECHNOLOGY, 206(21):4462-4465(2012)