This paper addresses various deformation mechanisms of unirradiated and irradiated V-4Cr-4Ti in terms of Ashby-type deformation maps. Tensile tests were carried out in the temperature range of 950–1150°C at strain rates of 10-3, 10-4, and 10-5 s-1. In addition, published tensile data and thermal creep data before and after irradiation were analyzed to provide further information about the materials' deformation behavior. The extensive tensile database for V-4Cr-4Ti was used to determine the key material parameters in physically-based constitutive equations. The resultant constitutive equations were employed to construct the Ashby deformation mechanism maps for unirradiated and irradiated V-4Cr-4Ti. Using the data analysis from the tensile tests, deformation map predictions at slow strain rates were obtained and verified by comparison with the experimental creep results. The methodology was successful in identifying the operating deformation mechanisms for a given temperature, stress, or strain rate condition. The study examines the limitations of extending creep data and analysis into new operating regimes, particularly where different deformation mechanisms are involved. The application of deformation mechanism maps in creep testing and data analysis is discussed.