The phase transition of Ca-deficient hydroxyapatite (CDHA) with citric acid (Cit) coordination was investigated. Cit promoted the substitution of K⁺ ions into CDHA to generate the HA phase. The K⁺-doping increased the phase transition temperature of CDHA, providing the transition to β- and α-tricalcium phosphates at higher temperatures. These results suggest controllable phase transition via Cit addition.