Novel visible-light-induced Co3O4-g-C3N4 heterojunction photocatalysts were synthesized via a facile mixing-and-heating method. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), ultraviolet-visible diffuse reflection spectroscopy (DRS), electron spin resonance (ESR) and photoluminescence spectroscopy (PL). The heterojunction photocatalysts exhibit a significantly enhanced photocatalytic activity in degrading methyl orange (MO). The optimal Co3O4 content with the highest photocatalytic activity was determined to be 0.2 wt%. The synergetic effect between Co3O4 and g-C3N4 plays an important role in promoting photo-generated carrier separation. The ESR and PL results reveals that the enhanced photocatalytic activity of Co3O4-g-C3N4 was mainly due to the superior amount and longer lifetime of oxidative radicals (O-.(2)-), as well as the efficient separation of charge carriers. Possible mechanism is proposed for the high photocatalytic activity of heterojunction structures, to guide the design of photocatalysts. (C) 2013 Elsevier B.V. All rights reserved.