The pi-conjugated structure of graphitic carbon nitride (g-C3N4) is particularly vital to many photocatalytic reactions. Herein, the hybrid structure of tri-s-triazine unit in g-C3N4 framework is chemically analyzed and expounded according to the hybrid orbital theory. The localized pi-conjugated structure of g-C3N4 is also monitored by the electron paramagnetic resonance (EPR) or electron spin resonance (ESR) technique. The experimental results indicate that this pi-conjugated structure is attributed to the orbital overlapping of the hybrid carbon and nitride atoms in their 2p(z) orbits. Unlike graphene with the nonlocalized pi-conjugated structure, this orbital overlapping in the whole two-dimensional plane of g-C3N4 is separated by the electrons pairs in 2p(z) orbits of the bridging nitride atoms, leading to the localized pi-conjugated structure. Therefore, the g-C3N4 exhibits the typical features of a semiconductor with band gap and visible-light response. Meanwhile, the EPR or ESR technique can be acted as the ideal tool to indirectly evaluate the yield of photoelectrons by detecting the superoxide radicals (center dot O-2(-)) in g-C3N4-based photocatalytic reactions.