Chandra X-ray Observatory (CXO) -- the third of NASA's Great Observatories -- has now been successfully operated for four years and has brought us fruitful scientific results with many exciting discoveries. The major achievement comparing to previous X-ray missions lies in the heart of the CXO -- the High Resolution Mirror Assembly. Its unprecedented spatial resolution and well calibrated performing characteristics are the keys for its success. We discuss the effective area of the CXO mirrors, based on the ground calibration measurements made at the X-Ray Calibration Facility in Marshall Space Flight Center before launch. We present the derivations of both on-axis and off-axis effective areas, which are currently used by Chandra observers.