Measurement error for echo intensity of weather radar mainly results from radar hardware and will have a direct influence on the accuracy of quantitative precipitation estimation. This paper introduces calibration principles for operational weather radars of China and USA. In addition, it compares two expression forms for weather radar equation using Syscal and dBZ0 as calibration constant respectively. Moreover, it analyzes factors that affect measurement accuracy of echo intensity, namely, variations in transmitter power and receiver gain. What is more, taking CINRAD radars（Beijing CINRAD/SA and Hangzhou CINRAD/CA） and WSR88D（KTLX and KICT）radars as examples, it presents practical variation curves and analysis results for calibration constants. By comparison, it can be found that calibration process using dBZ0 as calibration constant is more stable and hence is helpful for improving radar data quality.