High-performance photocatalysts should have highly crystallized nano crystals (NCs) with small sizes, high separation efficiency of photogenerated electronhole pairs, fast transport and consumption of photon-excited electrons from the surface of catalyst, high adsorption of organic pollutant, and suitable band gap for maximally utilizing sunlight energy. However, the design and synthesis of these versatile structures still remain a big challenge. Here, we report a novel strategy for the synthesis of ultrasmall and highly crystallized graphene-ZnFe2O4 photocatalyst through interface engineering by using interconnected graphene network as barrier for spatially confined growth of ZnFe2O4, as transport channels for photon-excited electron from the surface of catalyst, as well as the electron reservoir for suppressing the recombination of photogenerated electron-hole pairs. As a result, about 20 nm ZnFe2O4 NCs with highly crystallized (311) plane confined in the graphene network exhibit an excellent visible-light-driven photocatalytic activity with an ultrafast degradation rate of 1.924 x 10(-7) mol g(-1) s(-1) for methylene blue, much higher than those of previously reported photocatalysts such as spinel-based photocatalysts (20 times), TiO2-based photocatalysts (4 times), and other photocatalysts (4 times). Our strategy can be further extended to fabricate other catalysts and electrode materials for supercapacitors and Li-ion batteries.