This paper describes the terrain height measurement capability of synthetic aperture radar with an interferometer assuming the radar operates on a satellite. The interferometer phase measurement uncertainty due to additive thermal noise and multiplicative speckle noise, which determine the upper limit of the height measurement accuracies of this radar system, has been obtained by numerical simulation. The height measurement accuracies attainable on a relatively flat terrain areas are of roughly the same order as range resolution when C- or X- band radar is operated at an altitude of 300 km.