A new method of precision power measurement to 1 kW lasers, where a high-reflection coated beam splitter is used as a high-power optical attenuator, and this attenuator is combined with an isothermal calorimeter. The calorimeter is a double-configuration consisting of an absorbing unit using a disk absorber having a short time constant and a Peltier cooling unit which are operated simultaneously. The principle of the measurement is based on the dc substitution method. The ratio of the substituted laser power to the dc power is determined for the absorbing unit. The design of basic construction is described.