In this work, the effect of crushing, temperature, oxalic acid concentration and stirring speed on the leaching of spent Ni-Mo HDS catalyst was investigated. Spent Ni-Mo HDS catalyst was roasted at 500 degrees C under dynamic air atmosphere for 4 h prior to leaching experiments. It was found that roasted spent Ni-Mo HDS catalyst included mainly Al2O3, MoO3, NiMoO4 and AlPO4. Leaching experiments were performed using 5 g of uncrushed roasted Ni-Mo HDS catalyst, 1 L of 0.25, 0.50, 0.75 and 1 M H2C2O4 solutions, 25, 40, 55 and 70 degrees C temperatures and 200, 250 and 300 rpm stirring speeds. Leaching behavior of Mo, Al, P and Ni was determined. It is seen that H2C2O4 can be used successfully for the leaching of roasted HDS catalyst. More than 90% of the molybdenum and 80% of Nickel is extracted from the experiments carried out using 300 rpm stirring speed, 1 M H2C2O4 and 40 degrees C.