Calcium phosphate slurry processing is a novel chemical treatment that leads to the formation of surface layer including calcium phosphate in one-step. The processing involves burying a metallic substrate in a slurry-like agent comprised of calcium phosphate powder and distilled water followed by heating the slurry including the substrate in air. In the present study, we challenged to apply this processing to bioinert zirconium (Zr) substrates to enhance their bioactivity. Treating by the slurry processing, a zirconium oxide (ZrO2) layer including tiny hydroxyapatite (HAp) particles was formed on a Zr surface, and we revealed an increase in the amount of HAp particles and the thickness of the ZrO2 layer with increasing heating-temperature. When soaked in a simulated body fluid, calcium phosphate precipitated only on the treated Zr surfaces regardless of heating-temperature, whereas the amount of precipitations was highest when treated at 473 K. As expected, calcification of osteoblast-like cells on the Zr surfaces treated at 473 K was significantly promoted as compared to untreated. Additionally, no cytotoxic effects were observed on the treated surfaces. Based on these results, we conclude that the slurry processing is a simple one-step technique for enhancing bioactivity of Zr substrates, which can be applied as an industrial technique.