We report the highly efficient experimental photocatalysis by the N vacancy in graphic carbon nitride with tris-s-triazine structures (g-C₃N₄), where 2-propanol is photocatalyzed to acetone. The rate of reaction is increased up to 9.6 times by N defect. Experimental and theoretical analysis reveals that the N vacancy is localized on the side N atom of tris-s-triazine units, which results in the highly efficient photocatalysis. The N vacancy is an essential factor for determining the defecting element and vacancy site for rational design of the highly efficient photocatalyst, using two-dimensional (2D) semiconductor materials on the nanoscale. Our results can promote a deeper understanding of the contribution of the defecting element and the vacant site to photocatalysis.