The objective is to identify the micromechanisms of ductile crack advance, and isolate the key microstructural and material parameters that affect these micromechanisms and fracture toughness of ductile structural materials. Three dimensional, finite element, finite deformation, small scale yielding calculations of mode I crack growth are carried out for ductile material matrix containing two populations of void nucleating particles using an elasto-viscoplastic constitutive framework for progressively cavitating solid. The larger particles or inclusions that result in void nucleation at an early stage are modeled discretely while smaller particles that require large strains to nucleate voids are homogeneously distributed. The size, spacing and volume fraction of inclusions introduce microstructure-based length-scales. In the calculations, ductile crack growth is computed and fracture toughness is characterized. Several features of crack growth behavior and dependence of fracture toughness on microstructural and material parameters observed in experiments, naturally emerge in our calculations. The extent to which the microstructural and material parameters affect the micromechanisms of ductile crack advance and, hence, the macroscopic fracture toughness of the material is discussed. The results presented provide guidelines for microstructural engineering to increase ductile fracture toughness, for example, the results show that for a material with small inclusions, increasing the mean inclusion spacing has a greater effect on fracture toughness than for a material with large inclusions.