The cementing efficiency factor (k-value) of concrete containing thermally modified fly ash (TMFA) was calculated according to the CEN method based on the concept of the equivalent water-cement ratio, and the estimation equation for the k-value was expressed as a function of the age and water-to-powder ratio of the concrete, the TMFA replacement ratio and the specific surface area. This estimation equation was applied to the k-value of domestic and foreign FA concretes and was found to be valid for domestic FA concrete. The strength of TMFA and domestic FA concrete was also adequately expressed by the estimated k-value.