The photocatalytic performance of the star photocatalyst g-C₃N₄ was restricted by the low efficiency because of the fast charge recombination. The present work developed a facile in situ method to construct g-C₃N₄/g-C₃N₄ metal-free isotype heterojunction with molecular composite precursors with the aim to greatly promote the charge separation. Considering the fact that g-C₃N₄ samples prepared from urea and thiourea separately have different band structure, the molecular composite precursors of urea and thiourea were treated simultaneously under the same thermal conditions, in situ creating a novel layered g-C₃N₄/g-C₃N₄ metal-free heterojunction (g-g CN heterojunction). This synthesis method is facile, economic, and environmentally benign using easily available earth-abundant green precursors. The confirmation of isotype g-g CN heterojunction was based on XRD, HRTEM, valence band XPS, ns-level PL, photocurrent, and EIS measurement. Upon visible-light irradiation, the photogenerated electrons transfer from g-C₃N₄ (thiourea) to g-C₃N₄ (urea) driven by the conduction band offset of 0.10 eV, whereas the photogenerated holes transfer from g-C₃N₄ (urea) to g-C₃N₄ (thiourea) driven by the valence band offset of 0.40 eV. The potential difference between the two g-C₃N₄ components in the heterojunction is the main driving force for efficient charge separation and transfer. For the removal of NO in air, the g-g CN heterojunction exhibited significantly enhanced visible light photocatalytic activity over g-C₃N₄ alone and physical mixture of g-C₃N₄ samples. The enhanced photocatalytic performance of g-g CN isotype heterojunction can be directly ascribed to efficient charge separation and transfer across the heterojunction interface as well as prolonged lifetime of charge carriers. This work demonstrated that rational design and construction of isotype heterojunction could open up a new avenue for the development of new efficient visible-light photocatalysts.