This paper gives a brief account of a calorimeter developed at the National Bureau of Standards for the measurement of microwave power with relatively high accuracy (better than 1 percent) and at low (milliwatt) power levels. The calorimeter is identifiable as an electrically (dc) calibrated, aneroid micro‐calorimeter of the Joule twin type. A bolometer mount assembly serves as a wave‐guide termination for the absorption of microwave power and constitutes the calorimetric body whose temperature rise is observed. This arrangement enables the use of a novel indirect measurement technique and tends to insure calorimetric equivalence of dc and rf heating. The equivalence error is investigated with the aid of heat flow analysis and auxiliary experiments, and an upper bound for the error is assigned.