The US-Japanese ASTRO-E observatory, is scheduled to be launched in early 2000. ASTRO-E carries four X-ray CCD detectors and a hard X-ray detector. The CCDs are located at the focus of grazing incidence X-ray mirrors and will primarily provide imaging over 0.4–12 keV bandpass. ASTRO-E also carries the XRS microcalorimeter X-ray detector. A platinum X-ray mirror will focus X-rays onto a 32-element array of microcalorimeter pixels for high-throughput, high-resolution spectroscopy with limited spatial resolution. The mean measured energy resolution of the XRS flight model detector is about 12 eV at 6 keV for the nominal operating temperature of 65 mK. We present results from our calibration of the XRS flight model detector. We describe the methods used to determine the spectral redistribution of the detector and the overall detection efficiency. ©1999 American Institute of Physics.