A spherical blue CoAl2O4 pigment powder commonly used for coloring ceramic products has successfully been synthesized by co-precipitation of Co2+ and Al3+ ions in Pickering emulsion assisted with a hydrothermal process, and the formation mechanism of the as-prepared powder has briefly been discussed. The effects of pH condition on the crystalline phase and color tone of the pigment powder were investigated at 280 degrees C by varying the pH value of the reaction system. Then, under an optimized pH condition to produce blue, the pigment powder was synthesized still hydrothermally at different temperatures (210, 240, 270 and 300 degrees C) to assess the effect of hydrothermal reaction temperature on the pigment particle morphology. The resulting pigment powders were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), CIELAB colorimetric analysis (CIE-L*a*b*), laser particle size analysis (LPS), and N-2-adsorption measurement (BET). FE-SEM and TEM observations indicated that the pigment powders of spherical shape obviously were constructed by a core-shell double-layer structure, the radii of which all were similar to 150 nm or so with a shell of similar to 25 nm in thickness. XRD results demonstrated that the pigment powders that were obtained from the aforementioned method under a wide range of pH values invariably contained basically crystalline CoAl2O4 of high purity. CIE-L*a*b* data suggested that the pigment powders at 280 degrees C and pH <= 11.2 appeared different chromas of bluish colors compared with that displaying black which was obtained at pH = 12.3. LPS and BET results showed that the CoAl2O4 pigment powder prepared under the urea concentration of 3.6040 g for 24 h at the hydrothermal temperatures of 240 degrees C exhibited a narrow particle size distribution, the specific surface area of which was evaluated to be similar to 56.4 m(2)/g. (C) 2018 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.