Most of the targeted heavy metals (HMs) in sewage sludge ended up in solid residues after pyrolysis and gasification, such as Cu, Zn, Cr, Ni and Pb, which their residual rates were over 70%. The bioavailability of the HMs in residues decreased despite their concentrations being higher compared with in sludge. The potential eco-toxicity of each HM were ceased after pyrolysis and gasification of sludge. It was particularly noted that HMs in steam gasification residue exhibited the lowest contamination levels. In addition, the modification of pyrolysis temperature could pose a significant reduction on contamination level and environmental risk of HMs.