Modern precision measurements of time and frequency are the most accurate measurements among measurements of other physical quantities. They are impossible without the use of highly stable reference frequency sources (frequency standards) and frequency comparators.The paper presents the main results of research in the calibration of precision frequency comparators using high-precision frequency standards. The structural scheme and model of measurements, and also features of definition of the budget of the measurement uncertainty at calibration are described.The influence of the most significant influence values on the accuracy of measurement results is analyzed. The content of quantitative and qualitative indicators of corrections that must be taken into account during calibration to achieve the highest measurement accuracy is revealed. Practical results of researches of instability of the frequency entered by the comparator at zero difference of frequencies of input sinusoidal signals at frequency of 5 MHz from the cesium generator in a frequency bandwidth of 3 Hz on time intervals of 1 s, 10 s, 1 hour, and 1 day are resulted.Practical results include numerical values and plotted on the main indicators of frequency instability, such as: increase in phase difference between signals, relative frequency difference between signals, root mean square relative frequency deviation, root mean square relative two-sample frequency deviation (Allan deviation), and power spectrum of relative deviations of signal frequency.Uncertainty budgets were compiled and the results of calibration of the frequency comparator Ч7-308А/1 at the time intervals of measurements of 1 s, 10 s, 1 hour, and 1 day are given. The main advantages of calibration of comparators with the minimum difference of input frequencies and the optimal number of measurements at each time interval of measurement are shown.It is expedient to use the described method by each metrological laboratory which carries out calibration of frequency comparators.