The single-chain distribution in a poly(styrene-block-isoprene) block copolymer in the bulk state was studied by the combined method of transmission electron microtomography and three-dimensional mesoscale computer simulations. The local frustration of the block chains on a nanoscale can be analyzed using the size of the gyration radius, Rg. Two representative morphologies in the lamellar morphology have been examined, i. e., flat and bent lamellae. The Rg obtained from the proposed new method gave reasonable agreement with the previously measured values. It was shown that the single polymer chains in the bent lamella is more stretched out than those in the flat lamella. This method provides a new insight into understanding the self-assembled structure in the bulk microdomains.