Nickel hydroxide sludge samples used in this research were obtained from a sand dring bed of chemical wastewater treatment plant for a nickel eletroplating process containing 43% nickel. Its pH was around 8. This research consist of two parts which in both of shake flask and column. The first part of this research is to study the leaching efficiency of nickel hydroxide sludge by sulfuric acid. The experiment in shake flask was carried out using nickel hydroxide sludge which containing nickel ion at 10 g/L by varying the concentration sulfuric acid at 0.01, 0.05, 0.1, 0.15, 0.5, 1 and 5N. The result indicated that the concentration of sulfuric acid at 5, 1 and 0.5N can achieve 100% leaching efficiency within 24, 48 and 84 hours respectively. The concentration of sulfuric acid at 0.5N can consume 1.06 g.H2SO4per g. sludge. The experiment in column was carried out using 500 g. of nickel hydroxide sludge in a column 5 cm. in diameter with 50 cm. height by using 1N H2SO4 at flow rate of 15 ml/cm2-hr. It can leach nickel at 13.56% within 3.5 hours. The other part of this research is bacterial leaching of nickel from nickel hydroxide sludge by Thiobacillus ferrooxidans and Thiobacillus thiooxidans. The experiment in shake flask was carried out using nickel hydroxide sludge which containing 10 g/L of nickel ion in different culture with bacterial. The adapted strain of both types of Thiobacillus spp. have 47% efficiency in leaching nickel which cooresponds to the sulfuric acid concentration of 0.15N and is higher than for nonadapted bacteria and the sterile control, respectively. The experiment in column was carried out nickel hydroxide sludge in the same size of column as the first experiment using an adapted strain of T. ferrooxidans and T. thiooxidans in 9K medium and thiomedium, respectively. The optimum condition for T. ferrooxidans was obtained by varying flow rate at 5, 10, 15 and 20 ml/cm2-hr, the inoculum amount of T. ferrooxidans at 10% and 20%(v/v); ferrous iron in 9K medium at 4, 10, 20, 30, 40 and 50 g/L; and quantity of sludge at 250 and 500 g. per column. The result indicated that the optimum condition were at a flow rate of 15 ml/cm2-hr, with 20%(v/v) innoculum and the concentration of ferrous iron at 30 g/L, under which T. ferrooxidans can leach nickel from nickel hydroxide sludge 250 g. per column at 32.90% whithin 150 days. The controlled pH of the aforemention step is between 2.5-3.0. The efficiency of nickel leaching from the sludge mixed with sulfer 0.2 g./g. sludge packed in column with T. thiooxidans was higher, with 37.92% leaching whithin 150 days at a flow rate of 15 ml/cm2-hr, with 20%(v/v) innoculum, 250 g. sludge per column and a controlled pH of 1.5-2.0 at the column inlet. The cost of bacterial leaching which in both of shake flask and column was found to be lower in chemical leaching by sulfuric acid. The cost of leaching nickel from nickel hydroxide sludge by sulfuric acid, T. ferrooxidans and T. thiooxidans. which in shake flask experiment were 19, 99 and 160 baht per kg. of sludge, respectively and which in column experiment were 97, 174 and 163 baht per kg. of sludge, respectively.