[en] Hall-Petch sigmasub(S)(dsub(-1/2)) and Mott-straw sigmasub(B);(dsub(-1/2)) relations have been examined for heterogeneous molybdenum alloy Mo-i.07%C recrystallzed to a varying grain size (from 20 to 120 microns). It is shown that in the region of the studied structural states, the Hall-Petch relation is met with constant parameters, sigmasub(O) and Ksub(u) independent of the character of fracture; parameters sigmasub(OB) and Ksub(p) of the Mott-Straw relation change with transition from ductile transcrystallite to mixed fracture mechanism. The controlling stage of fracture is the process of crack propagation