The effects of aging temperature and grain size on the formation of serrated grain boundaries have been investigated in an AISI 316 stainless steel. Grain size increased slightly over aging temperature ranges of 650-870 degrees C, resulting in predominantly serrated grain boundaries. However, when the temperature exceeded 880 degrees C, the grain size significantly increased, and grain boundary serration was not observed. The initial grain size also had an influence on the occurrence of grain boundary serration. For specimen having a large initial grain size of about 200 mu m, no serrated grain boundary formed after aging treatment at 760 degrees C. Serrated grain boundaries were observed when "normal" initial grain sizes 55 mu m were employed. It was found that the frequency of low angle boundaries markedly increased as the initial grain size increased from 55 to 200 mu m. From the results obtained, it is possible to describe that the grain boundary serration could be considered as a spontaneous reaction that aims to reduce the total free energy of the system, and form a new interface of lower free energy. We proposed that the competition between grain growth and grain boundary serration during aging treatment reduces the total free energy of the alloy system: at temperatures exceeding 880 degrees C, the dominant process is the grain growth, while grain boundary serration predominates over the range of 650-870 degrees C. (C) 2012 Elsevier B.V. All rights reserved.