Strengthening by refinement of lamellar thickness was studied at room temperature on dual phase Ti-39.4mol%Al alloy over a wide range of average lamellar thickness λ from 850 to 20 nm. The relation between yield stress σy and λ was examined, paying special attention to the change in lamellar boundary structure. The γ/α2 lamellar boundaries in the alloy are found to be perfectly coherent in thin lamellar structure formed at low aging temperatures. In thick lamellar structure formed at high aging temperatures, misfit dislocations were introduced to relieve the lattice misfit and are found on the lamellar boundaries. Both thin lamellae with coherent boundaries and thick lamellae with dislocated ones are present in a lamellar structure formed at an intermediate aging temperature. The critical thickness of γ lamella for the introduction of misfit dislocations is about 50 nm. The dislocated boundaries render a high resistance to dislocation motion across the boundaries. A Hall-Petch relation holds in the range of λ > 170 nm, and the Hall-Petch slope takes a large value corresponding to the high boundary resistance. The coherent boundaries provide a relatively low resistance. Another σy-λ correlation typical of the coherent boundary appears in the range of λ < 100 nm. The yield stress saturates to an upper limit of 1 GPa at λ = 70 nm. The transition from the property of dislocated boundary to that of coherent boundary proceeds with an increase in the density of the coherent boundaries within the range of λ = 170-100 nm. © 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.