Abstract Porous nanoflower-like, micropancake-like and microflower-like Co3O4 micro/nanostructures were synthesized by a template-free aqueous solution route combined with subsequent thermal treatment. Techniques of X-ray diffraction, scanning electron microscopy, thermogravimetric-differential thermal analysis, and transmission electron microscopy were used to characterize the structure and morphology of the products. The experimental results show that three kinds of morphologies of cobalt precursors can be achieved by addition of ammonia at different temperatures. The corresponding Co3O4 hierarchical micro/nanostructures were obtained after 500°C calcinations. In addition, the obtained porous Co3O4 hierarchical micro/nanostructures were used as catalyst to photodegrade Rhodamine B, methylene blue, p-nitrophenol, eosin B, and methyl orange. Compared with porous Co3O4 micropancakes, the as-prepared porous Co3O4 nanoflowers and microflowers exhibit higher catalytic activities due to their large surface areas and porous hierarchical structures. The photocatalytic reaction rate constant of the porous Co3O4 nanoflowers in photocatalytic decomposition of Rhodamine B under UV light is calculated as 0.0828min−1.