ZnAl-LDH was successfully modified with different loadings of g-C3N4 by a simple in situ crystallization technique. The rate constant of C3N4-18/ZnAl-LDH was more than five times and almost two times than that of pure ZnAl-LDH and C3N4 respectively. The enhanced activity of C3N4/ZnAl-LDH composites could be ascribed to the synergistic effect between two components of the composites.