A novel g-C3N4/Bi4Ti3O12 photocatalyst with the p-n heterojunction structure was prepared through ball milling. The morphologies, structures, and optical properties of the photocatalyst were comprehensively characterized. The transmission electron microscopy (TEM) images and the X-ray photoelectron spectroscopy (XPS) showed the interfacial interaction between g-C3N4 and Bi4Ti3O12, indicating that a heterojunction between g-C3N4 and Bi4Ti3O12 was formed during ball milling. In addition, the optimum activity of the coupling semiconductor is higher than that of individual g-C3N4 and Bi4Ti3O12 for the degradation of acid orange-II(AO-7). The enhanced photocatalytic activity could be ascribed to the p-n junction of g-C3N4/Bi4Ti3O12 with strong oxidative ability and efficient charge separation. The highest activity was obtained in the g-C3N4/Bi4Ti3O12 p-n heterojunction using a composite of 10 wt.% g-C3N4 and 90 wt.% Bi4Ti3O12 by ball milling in 16 h. The removal of AO-7 was not only mainly initiated by valence band holes (hi), but also affected by O-center dot(2)-, which was verified by the effects of scavengers. The mechanism underlying the observed photocatalytic activity was also described based on the semiconductor energy band theory and the formation of an internal electrostatic field. The probable degradation pathways were proposed and discussed according to the primary degradation products identified by GC/MS. (C) 2016 Elsevier B.V. All rights reserved.