A g-C3N4@ZnO heterojunction is demonstrated using atomic layer deposition (ALD) of ZnO. A specially designed rotary reactor was used to maintain mechanical dispersion of g-C3N4 powder during the ALD process. Stable, uniform, and intimate heterojunctions between g-C3N4 and ZnO were produced, which induced effective charge separation; thus, the photocatalytic activity of the composites was enhanced. The photocatalytic performance was evaluated by the degradation of methylene blue dye. The photocatalytic reaction rate constant of the optimal g-C3N4@ZnO with five ALD cycles was five times and two times higher than those of pristine g-C3N4 and gC(3)N(4)@TiO2 composite, respectively. Furthermore, the photocorrosion of ZnO was inhibited by coupling with gC(3)N(4), which was confirmed through cyclic photo-degradation with three consecutive dye degradation tests. The synergistic effects of the g-C3N4@ZnO heterojunction, enhanced photocatalytic activity and photocorrosion resistance were proven.