This study focused on the behavior of phosphorus ( P) and its effect on cadmium (Cd) immobilization in sludge during hydrothermal treatment (HTT). The results showed that, after HTT other forms of P in sludge were almost completely converted into apatite P (AP) and the eco-toxicity and leachability of Cd was sharply decreased, signaling the immobilization effect of HTT on Cd in the sludge. Hydroxyapatite (HAP) addition showed a synergetic effect with HTT on Cd risk reduction, implying the immobilization effect of HTT on Cd could be partly attributed to the strong binding capacity of phosphorus with Cd. P content was found to have a strong relationship with the existing forms of Cd during HTT. The present results suggest that HTT with HAP addition could be a promising method for the safe disposal of Cd-contaminated sludge. (C) 2014 Elsevier Ltd. All rights reserved.