The structural defects in g-C3N4 play an important role for photocatalytic activity of g-C3N4 for hydrogen production. We conducted a study on the relationship between the surface defect concentration and the photocatalytic activity having different modification time for the hydrogen production. The ball milling process for the elimination of the defects was adopted. The amino group created by incomplete polymerization of g-C3N4 may be considered as the defects, and the improved photocatalytic activity was measured after the modification. The enhancement can be ascribed to both enlarged surface area and reduced surface defect concentration. In order to identify the major factor for the enhancement of hydrogen production rate, the linear plot for photocatalytic activity versus the surface area and the degree of polymerization was obtained, respectively. Finally, it was, identified that the reduced surface defect concentration is a major factor for the enhancement of the photocatalytic activity. (C) 2016 Elsevier B.V. All rights reserved.