Cobalt-based heterogeneous cocatalysts are important substitutions of noble metal cocatalysts in many important commercial chemical processes, but their efficiency is extremely low on a per metal atom basis, because only the atoms located at surface active-sites participate in the chemical reaction. Thus, cocatalysts with small cluster dispersions are highly desirable to maximize the amount of active-sites and enhance the per atom efficiency. Here, we report the synthesis of sub-nanometer Co3O4 clusters which are anchored to 2D ultrathin TiO2(B) nanosheets, as a cocatalyst for H2 evolution reaction (HER). It was found that the conduction type of Co3O4 clusters turns from P-type to N-type, and the heterojunction band structure between TiO2(B) and Co3O4 clusters changes from type II to type I, when the cluster size is reduced from nanometer scale to the sub-nanometer scale. With a suitable energy band matching between TiO2(B) and sub-nanometer Co3O4 clusters, the electrons generated in TiO2(B) during the photocatalytic process reduce the Co ions into metallic Co atoms, which produce excellent photocatalytic stability and extremely high HER efficiency comparable to that of the noble Pt cocatalyst.