The structure and mechanical properties of a large-size semiproduct from alloy VST5553 treated in the BASCA mode (beta annealed, slow cooled, aged) are studied by testing for impact bending and delayed fracture. The microstructures and fracture surfaces of specimens are investigated. To intensity the processes of decomposition of the β-phase in near-boundary regions an additional heat treatment including heating within 770 - 790°C and aging at a temperature of 550 - 600°C is suggested, which raises the mechanical characteristics under delayed fracture.