When a ductile material with a crack is loaded in tension, the deformation energy builds up around the crack tip and it is understood that at a certain critical condition voids are formed ahead of the crack tip. The crack extension occurs by coalescence of voids with the crack tip. The “characteristic distance” (Lc) defined as the distance b/w the crack tip & the void responsible for eventual coalescence with the crack tip. Nucleation of these voids is generally associated with the presence of second phase particles or grain boundaries in the vicinity of the crack tip. Although approximate, Lc assumes a special significance since it links the fracture toughness to the microscopic mechanism considered responsible for ductile fracture. The knowledge of the “characteristic distance” is also crucial for designing the size of mesh in the finite element simulations of material crack growth using damage mechanics principles. There is not much work (experimental as well as numerical) available in the literature related to the dependency of “characteristic distance” on the fracture specimen geometry. The present research work is an attempt to understand numerically, the geometry dependency of “characteristic distance” using three-dimensional FEM analysis. The variation of “characteristic distance” parameter due to the change of temperature across the fracture specimen thickness was also studied. The work also studied the variation of “characteristic distance”, due to the change in fracture specimen thickness. Finally, the ASTM requirement of fracture specimen thickness criteria is evaluated for the “characteristic distance” fracture parameter. “Characteristic distance” is found to vary across the fracture specimen thickness. It is dependent on fracture specimen thickness and it converges after a specified thickness of fracture specimen. “Characteristic distance” value is also dependent on the temperature of ductile material. In Armco iron material, it is found to decrease with the increase in temperature.