Annotation: The article describes the works on researching the effect of indentation time in the measurement of the hardness using Rockwell scales. The sensitivity coefficients are determined at different times of indentation applied to the hardness blocks. The dependence of the measured hardness from the load application time is obtained. The influence of the load time application to expanded uncertainty in the measurement of hardness in the range of 20-30 HRC, 40-50 HRC, 60-70 HRC and 80-86 HRA is shown. The works were carried out on the national primary standard of Ukraine of hardness units (DETU 02-04-99).