The formation process and product control are very important in material synthesis. In this study, a facile one-pot hydrothermal method was used to prepare Co3O4 and CoOOH. H2O2 was used to modulate the formation process and control the final product by changing its concentration. The crystalline structures and morphologies of the as-prepared products were characterized by X-ray diffraction (XRD), Raman spectra, and scanning electron microscopy (SEM) techniques. It was found that the concentration of H2O2 influenced not only the phase of the final products but also their morphologies. The influences of H2O2 concentration on the precursor formation and the reaction path have been revealed. At a low concentration of H2O2 (5 wt %), the formed precursor is Co(CO3)(0.5)(OH)center dot 0.11H(2)O, which can be directly transformed into Co3PO4 upon increasing the hydrothermal time. At a medium concentration (15-20 wt %), the formed precursor and the final product are all CoOOH. At a high concentration (30 wt %), the formed precursor is CoOOH, and the final product is Co3O4. H2O2 plays the role of oxidant agent at the initial stage or reducing agent at the subsequent stage. This study offers a H2O2-concentration modulating method for the formation of Co3O4 and CoOOH.