The bioleaching of heavy metals from fly ash was performed by Thiobacillus thiooxidans MET isolated from the enrichment culture of an anaerobically digested sludge. The effect of solid concentrations on the efficiency of metal leaching was studied in shaken flasks. In the range of solid concentrations 20 g.L­$^1$to 100 g.L­$^1$T. thiooxidans MET oxidized S$^{0}$ to sulfate without any lag period. The final pH of slurry solution was decreased to below pH 1, and the final oxide-redox potential (ORP) was increased to over 420 mV in the solid concentrations below 100 g.L­$^1$. However, the initial lag period of 4 to 8 days was required to obtain the pH reduction and ORP increase of the slurry solutions in the range of solid concentrations 150 g.L­$^1$to 300 g.L­$^1$. The sulfur oxidation rate of T. thiooxidans MET in 20~100 g.L­$^1$solid concentrations was 0.70~0.75 g-S.L­$^1$ㆍ d­$^1$, but its sulfur oxidation activity was remarkably inhibited with increasing solid concentration over 150 g.L­$^1$. Increasing fly ash solids concentration in the range of solids concentration 20 g.L­$^1$ to 200 g.L­$^1$decreased the removal efficiency of Zn, Cu, Mn, Cr and Pb. The solubilization of heavy metals from fly ash was strongly correlated with the pH value of slurry solution. When the pH of slurry solution was reduced to 3, the solubilization process of Zn, Cu and Mn started, and their solubilization efficiency of Zn, Cu and Mn was progressively increased below pH 2. However, the solubilization process of Cr and Pb started at pH 2.5 and 2.0, respectively.