The low surface area, poor electrical conductivity, and rapid electron-hole recombination in bulk C3N4 limit its photocatalytic activity, which makes it challenging to improve the performance of bulk C3N4. Herein, an effective strategy is proposed to fabricate Co3O4/C3N4 heterojunctions (Co3O4 nanoparticles grown on C3N4 nanosheets), where bulk C3N4 is exfoliated to thin nanosheets. The bulk C3N4 precursor was synthesized with the hydrothermal treatment of melamine solution, and Co2+ ions were then inserted into the interlayer of the precursor through a vacuum-assisted intercalation process. Subsequently, the precursor was exfoliated to C3N4 nanosheets, and 15 nm Co3O4 nanoparticles were simultaneously formed using in situ thermal polycondensation. The Brunauer-Emmett-Teller (BET) specific surface area of the prepared heterojunction was 21 times higher than that of bulk C3N4, and thus more active sites were exposed on the surface of the heterostructure. Co3O4 nanoparticles contained oxygen vacancies, and the type-II transfer mechanism between these nanoparticles and C3N4 could be used to effectively separate photogenic carriers and improve the electron mobility. As expected, the heterostructure exhibited an excellent photocatalyzed degradation rate of 99.5% for methylene blue within 30 min (10 mg catalyst, wavelength >420 nm) under visible light irradiation, which was nearly three times higher than that of bulk C3N4. Electron paramagnetic resonance (EPR) analysis indicated that ˙O2- was the main reactive oxidizing species during the degradation process.