Crack nucleation mechanisms in two types of cold work tool steels were evaluated under monotonic and cyclic loading conditions. The effect of the microstructural constituents: the primary alloy carbides and the tempered martensite matrix, and their interaction, was identified through fractographic analysis and determination of mechanical properties such as the bending strength, σR, the fatigue limit, Δσfat, and the fracture strength of the primary carbides under static tensile stressing, σRC. The response under monotonic loading was found to be governed by the fracture of primary carbides. The cracks nucleated when the applied stresses were higher than the carbides fracture strength, accordingly depending on their properties, morphology and arrangement. Under cyclic loading, despite failure origins were located at the primary carbides and cracks emanating from them and propagating through the metallic matrix were evidenced, crack nucleation phenomena could not be explained as in monotonic loading since the applied stresses in fatigue were lower than the determined σRC. Carbides fracture was then probably caused by damage observed in the metallic matrix. Primary carbides acted as stress concentrators and strain localization was more likely occurring in the matrix around them. Thus, fatigue failure was attributed to the destabilization of the tempered martensite of the matrix, induced by the strain localization processes around carbides, which produced their breakage and gave rise to fatigue propagating cracks.