The nucleation of creep cavities was analysed using data from the literature for creep resistant low alloy and 12%Cr steels. The number of cavities per unit area was in most cases found to be a linear function of the creep strain, the cavity nucleation rate increasing linearly with increasing minimum creep rate. As a result, the cavity nucleation rate showed the same type of stress and temperature dependence as the strain rate, i.e. a power law function of the stress, where the stress is raised to a power close to the exponent in the creep rate equation (Norton's exponent). Simple models relating the number of cavities to strain and time as well as relating the cavity nucleation rate to time were derived. The theoretically estimated cavity numbers and cavity nucleation rate in relation to strain and time showed satisfactory agreement with the observations for both low alloy and 12%Cr steels.MST/3353