A general method for the calibration of milliwattmeters operating in the centimetric-wave region is described. Results of measurements at 3000 Mc/s on a thermistor bridge milliwattmeter are given and it is shown that within the present limits of experimental error (estimated as±4%) this instrument gives true power indications. The powers indicated by other forms of milliwattmeter, for equal power inputs, have been determined and the results are summarized. It is concluded that a number of different instruments may be relied upon to indicate true power withing reasonably small limits of error.