Abstract: The study on rock weathering attracts attention of geomorphologists, geologists and geotechnical engineers. Historic structures can be viewed as the exposure trials of the stone of which they are constructed. And, they represent a weathering experiment. Long-term weathering rates of sandstones are obtained for well-dated historic structures, that is, the bottom walls of the Ancient City of Pingyao (with 638 a exposure time) by using an especially designed movable instrument with high precision. The weathering depth and maximum weathering depth of the sandstone are 8.03~32.30 mm and 11.80~46.97 mm respectively. The difference of sandstone weathering rates in sample areas is exhibited between four aspects, which are higher in the south facing sandstone wall areas (0.0481 mm?a-1) than those in the west (0.0377 mm?a-1) and east faces (0.0286 mm?a-1), and the north facing sandstone wall areas have the lowest weathering rate (0.0220 mm?a-1). The reasons for the difference of weathering rate probably are the difference of insolations with four aspects.