This paper presents the results of studies on the true fracture stresses of the Al alloy 6101 with an ultrafine-grained (UFG) structure processed by equal channel angular pressing - conform (ECAP-C) method and the same alloy after artificial aging (AA state). The grain size in the UFG state was about 500 nm. Aging particles and Al3Fe particles were present in the structure of AA and UFG state. Strength and yield stress increase as a result of the formation of UFG structure. The true strain to failure in AA and UFG states of 6101 alloy, taking into account the measurement error, is the same. In this case, the true fracture stress of samples with the UFG structure is markedly higher than the true fracture stress of samples with the AA structure. An explanation for the increase in the true fracture stress of specimens with UFG structure based on a generalization of the Hall-Petch relation and the Zener-Strauss model, a criterion of pore formation on the particle, is proposed.