The difference of the experimental and the calculated creep rates of the steel grade X20CrMoV121 with M23C6 particles in temperature range 763–913 K was examined in terms of temperature, iron self‐diffusion rate, glide stress, and number of ferrite lattice vacancies. The Ashby‐Hornbogen creep equation is modified by adding a parameter that represents the effect of number of lattice vacancies; consequently, the difference of the experimental and the calculated creep rates is diminish to the level of average particles size assessment accuracy. The effect of vacancies content and mobility, as well as of glide stress on creep rate of steel X20 is examined in range of creep tests 550–640 °C. It is found that the effect of increase of vacancies content prevails. With inclusion of the corresponding parameter, the difference of calculated and experimental creep rate is diminshed to the accuracy of assessment of carbide particles.