In an investigation of the microbial community on the Ming Dynasty Wall at Nanjing, microorganisms were cultivated from the surfaces of city walls at Zhonghuamen, Xuanwu Lake and Pipa Lake city and analyzed by electron microscopy. It is found that lichens, mushrooms, actinomycetes, filamentous fungi, cyanobacteria, epithelic diatoms and Chlorococcum are present ubiquitously on the city wall. On bricks with no lichens and mosses, It is found epithelic diatoms (Achnanthes lauenburgiana Hustedt and Pinnularia borealis Ehrenberg), which coexist with filamentous fungi. Further study in an experiment of cocultivation cultured algae and brick powder demonstrated their weathering effect on the wall brick. This research not only suggested the bioweathering effect of the epithelic diatoms on silicate stones in a terrestrial epilithic environment, but also provided the oretical support for efforts to prevent the corrosive effect of algae on the Ming Dynasty wall.