A laser calorimeter in compliance with ISO 11551 was developed to measure the absolute absorptance of optical thin films. Under a typical condition the absorptance sensitivity of the calorimeter was better than 10-6 and the measurement error was estimated to be ～10%. For an uncoated fused silica substrate with 1-mm thickness, the measured absorptance was 3.4×10-6 and the sensitivity was ～1.5×10-7. The laser calorimeter was employed to measure the absorptance of highly reflective dielectric coatings prepared by two different coating techniques. The absolute absorptance of a 0° Ta2O5/SiO2 multilayer mirror prepared by the ion beam sputtering (IBS) technique was as low as 1.08×10-5, and the absorptance of a 45° HfO2/SiO2 multilayer mirror prepared by the ion-assisted deposition (IAD) technique was 6.83×10-5, respectively.