Geometric dimensionality and morphology largely affect the properties and functionalities of materials; however, simultaneously regulating them to realize synergic effects is a formidable scientific and technological challenge. Here we demonstrate an effective strategy to control the dimensionality and morphology of graphitic carbon nitride (g-C3N4) through heat treatment the melamine-cyanuric acid supramolecular precursors formed in water as a "green" solvent. By varying heat treatments, the three-dimensional (3D) hexagonal prism precursors could be transformed to 3D g-C3N4 loofah-like (CNl) architectures, ultrathin two-dimensional (2D)g-C(3)N(4 )nanosheets (CNs), and ordered one-dimensional (1D) g-C3N4 nanotube (CNt) array, respectively. The adsorbed melamine molecules on the surface of precursor and atmosphere in the transformation process, play a key role in determining the morphology of products. The resulting ultrathin 2D CNs have a porous structure, a small thickness (1.6 nm), a large surface area (208.8 m(2).g(-1)), and high conductivity, thus exhibiting higher hydrogen evolution rate (23.9 mu mol h(-1)) by 17.3 times than the bulk g-C3N4 (CN) under visible light irradiation. This strategy results in high-quality, ultrathin CNs at yields of similar to 10 wt% from raw material, much higher than those of previous reports (similar to 6 wt% from bulk CN). This work not only enriches our understanding of the relationship between geometric dimensionality, morphology and properties of photocatalytic nanomaterials, but also could be potentially useful for the design and growth of 1D or 2D flexible polymers for energy-related applications and beyond.