The preparation of blue pigment CoKPO4 was studied by grinding the mixture of reddish violet colored CoNH4PO4.H2O and K2CO3 (mixing molar ratio 1.0 : O. 5) in ball mill. The color of the ground materials was gradually tinged with blue with increasing grinding time, and after 16 days grinding changed to indistinct blue. After more days of grinding the vividness increased gradually without changing its hue. After 30 days the ground material showed most vivid blue. This vivid blue colored material was identified as CoKPO4 by X-ray diffraction measurement, etc. Grinding CoNH4PO4.H2O and K2CO3 independently, the crystallinity of the former disappeared somewhat, and the latter changed into K2CO3.1. 5 1120.