This research aimed at finding the factors with more impact on the bioleaching of a spent hydrocracking catalyst by thermophilic acidophilic archaea called Acidianus brierleyi and determining the optimal condition in order to reach the maximum metals recovery. Initially, four more significant factors affecting the bioleaching efficiency including pH, pulp density, inoculation and elemental sulfur concentration, were screened and identified among ten key factors using Plackett-Burman factorial design. Next, a central composite design was applied to obtain the optimum conditions for achieving the maximum efficiency of the bioleaching process. pH 1.6, pulp density 0.6% (w/v), inoculation 4% (v/v) and elemental sulfur concentration 4 g/L were determined as the best conditions to reach the maximum recovery of the process. The experimental recoveries in optimal conditions were 35%, 83% and 69% for Al, Mo and Ni, respectively. To better understanding the mechanism of the leaching reaction, the rate limiting step of the process was determined under the optimum conditions and it revealed that diffusion through product layer is the rate controlling step. (C) 2013 Elsevier B.V. All rights reserved.