The transformation process of cobalt hydroxide (Co(OH)₂) to cobalt oxides (Co₃O₄/CoOOH) in aqueous solution was studied in the temperature range of 50–90 °C. The crystalline structures and morphologies of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM) analysis. The OH⁻ : Co²⁺ ratio and aging time influenced the transformation process from Co(OH)₂ to final products. And the formation of hydrotalcite-like phase intermediate ([Co^II_1−xCo^III_x(OH)₂](NO₃)_x·nH₂O) is a crucial factor for synthesizing Co₃O₄. When the OH⁻ : Co²⁺ ratio was in the range of 1.0 : 1–1.8 : 1, α-Co(OH)₂ was oxidized to [Co^II_1−xCo^III_x(OH)₂](NO₃)_x·nH₂O at first, and then transformed to Co₃O₄ crystals. When the OH⁻ : Co²⁺ ratio was in the range of 2.0 : 1–50.0 : 1, without the appearance of soluble Co(OH)₄²⁻ ions in the reaction system before oxidation, fast conversion from α-Co(OH)₂ to β-Co(OH)₂ resulted in the formation of only CoOOH crystals, without the appearance of [Co^II_1−xCo^III_x(OH)₂](NO₃)_x·nH₂O or Co₃O₄ crystals.