High temperature steam gasification experiments were conducted to gasify sludge in an electrically heated tubular furnace at 650, 750, 850, 950, and 1050°C. The obtained results develop a mechanistic understanding on steam gasification of sludge from starting constituents to final products. The introduction of steam led to a steam reforming of the tar and CnHm compounds, and a partial gasification of the solid char and CH4, which contributed to the generation of rich H2 and CO. The steam gasification process can be divided into three significant stages. As temperature increased, a decreasing absorbance amount of C-Har and C-O and a large reduction of C = O were accompanied with an evident reduction of tar yield and an increasing gas yield, especially the increase of H2 and CO contents. The H2 and CO were the indicator for the occurrence of tar and CnHm steam reforming. In addition, more micro-pores generated in solid char and a sharp increase of Brunauer-Emmett-Teller (BET) surface from 24.2 m2g–1 to 75.4 m2g–1 showed the occurrence of char decomposition. An increasing content of polycyclic aromatic hydrocarbons (PAH) with 2 and 3 rings and a significant decrease of PAH with 4-6 rings in tar showed the tar cracking and reforming of non-condensable intermediates.