To determine the optimum leaching conditions controlling the leaching of petroleum ash residues, the theoretical thermodynamics factors were firstly deduced using the computer program HSC chemistry 5.1. The factors affecting the leaching efficiency, such as the acid type and concentration, helper leaching agent type and amount, leaching temperature, agitation time, solid/liquid ratio, and the effect of grain size, were studied. When applying these optimum leaching conditions more than 95.9, 98.9, 97.1, 97.8, 96.5 and 97.1% of Hf, Zr, U, V, Ni and Fe contents was leached respectively.