In high temperature design, the accumulation of creep strain during the primary stage has to be considered since most of the allowable design strain occurs in this stage. In this work, assuming that the creep rate in the transient regime can be given as a fraction of the steady state creep rate and function of the internal stress, a mechanism based model for primary creep has been derived. Taking into account that the apparent activation energy varies with the internal stress, which evolves with creep strain, an exponential form of the creep rate versus creep strain has been obtained. The proposed model for primary creep requires the identification of two material parameters only which are shown to be function of the applied stress and independent of temperature. The proposed model has been validated for high chromium steel P91.