An Altitude Engine Test Facility(AETF) was built at the Korea Aerospace Research Institute in October 1999 and has been being operated for altitude testing of gas turbine engines of 3,000 Ibf class or less. The AETF has been calibrated using several engines such as J69 of Teledyne Co. as a facility checkout engine. Uncertainty analyses on the air flow rate and thrust were performed using the test results, according to ASME PTC 19.1-1998. Several modifications on the facility and test method were made in order to improve the measurement uncertainty to a satisfactory level over the whole operating envelop. Spatial distributions of pressure and temperature were measured, sensors were substituted by more accurate ones, inlet duct was modified to refine the flow quality, and pressure control logic was revised to remove the cell pressure fluctuation. As a result, the uncertainty of the air flow measurement was improved by 0.1% over all the test conditions, and the net thrust measurement by up to 3%. The improved measurement uncertainties of air flow and thrust are 0.68～O.73% and 0.4～1.3%, respectively.