In this study, the immobilization and risk assessment of heavy metals (HMs) in sewage sludge (SS) during hydrothermal treatment combined with pyrolysis (HTP) was investigated. The experimental results showed that the immobilization of HMs in SS was significantly affected and that the eco-toxicity/risk of SS to the environment was reduced through HTP, in addition to the enhancement of dehydration and a reduction in the volume of the sewage sludge. After the HTP process, although the majority of HMs were accumulated in the biochar, the HMs could be transformed from bioavailable fractions to more stable fractions, such as the acid soluble/exchangeable fractions (F1) of Zn, Ni and Cd in the biochar, which decreased to less than 6.31%, 2.36% and 3.64%, respectively. Moreover, the leaching potential of HMs decreased after the HTP process. (C) 2016 Elsevier Ltd. All rights reserved.