Abstract: Small Punch (SP) tests have been carried out at room temperature on small specimens with six kinds of thicknesses. Experimental results show that the SP fracture energy and the fracture toughness increase with the thickness of the samples. The fracture surface displays the typical ductile fracture, and the outer surface of the half-sphere part cut off by the punch is full of bulgings around which there are micro cracks. From the micro mechanics of the energy dissipation, the ductile fracture toughness is deduced from the SP fracture energy theoretically. The fracture strain and the fracture energy density dJ*/dt criterions are introduced into the damage model, with which the Finite Element Analysis simulations get the results of the crack propagation process which agrees with the experimental results. Simulations with Gurson-Tvergaard-Needleman model explain the bulgings on the half-sphere part surface of different thickness.