Many instruments can be used to measure radio frequency (RF) and microwave power. The most accurate one is a power sensor with a meter. The accuracy of RF and microwave power measurement depends on the accuracy of power sensor calibration. This chapter provides calibration methods of RF and microwave power sensor with system setup, modeling, equations, and analyses in different representations, traceability and measurement uncertainty evaluations. From the simple direct comparison transfer method, to the different improvements, and then the general analytical models, the methods provided in this chapter are useful for the guided-wave power sensor calibration in frequency range of a few MHz to several hundred GHz.