Nanostructural engineering can achieve smart modulation of the physical and chemical properties of nanomaterials by changing their dimensions, morphologies, and shapes. Nanostructures of different dimensions affect the bactericidal activity of nano‐antibacterial materials. Herein, the nanostructures of the metal‐free antibacterial substance polymeric carbon nitride (PCN) were engineered to prepare. The PCN nanotubes, nanosheets, and microspheres exhibited defined dimensional and morphological characteristics from one‐dimensional (1D) to 2D to 3D. Nanostructuring did not change the crystallization and bonding of PCN, but it profoundly impacted the specific surface area, porosity, absorption spectrum, fluorescence spectrum, photoelectric response, and impedance. Unsurprisingly, the photocatalytic activity of PCN nanostructures in killing Escherichia coli appeared dimensional dependence. The low‐dimensional PCN nanostructures including nanosheets and nanotubes had significantly boosted bactericidal activity; the E. coli survival rates on them after 2 h of visible light irradiation were only 10.4 and 15.8%, respectively. The excellence of the activity can be attributed to the comprehensive effect of strong photo‐generated hole oxidation ability, long photo‐generated carrier lifetime, fast carrier migration rate, large surface area, and sharp shape. The implement of nanostructure engineering provided a promising way to enhance the antibacterial activity of PCN photocatalysts.