Based on petrological and geochemical analysis for core samples from the Middle Ordovician Majiagou submember-55 dolostones in Ordos basin, three main types of dolostones are recognized, including micritic dolostone, siltcrystal micritic dolostone and siltcrystal dolostone. The characteristics of them are described as follows:(a) The micritic dolostone is formed in euxinic-restricted and high-salinity environment by penecontemporaneous dolomitization, in which a small number of intercrystal mircopores develop. It is characteristic of micrite texture, garnet light in cathoduluinescence observation, lower degree of order and high content of Na, K and Sr elements. The average values of δ13C and δ18O are-1.23‰and-7.59‰respectively, and the ratio of 87Sr/86Sr is 0.70981. (b) Siltcrystal micritic dolostone is formed by post-penecontemporaneous reflux seepage dolomitization, in which intercrystal pores develop. It is characteristic of siltcrystal-fine micrite texture, dirtier, no or very dark light in cathoduluinescence, 0.75 of degree of order, significantly decreasing of the content of major and trace elements. The average values ofδ13C and δ18O are -0.43‰ and -7.29‰, and the ratio of 87Sr/86Sr is 0.70936. (c) Siltcrystal dolostone, in which intergranular dissolution pores and dissolution pores develop, shows the same feature as siltcrystal-micrite dolostone in cathoduluinescence but the degree of order is 0.81. The content of major and trace elements shows apparently abrupt change. The average values of δ13C and δ18O are -0.65‰ and -6.46‰ and the ratio of 87Sr/86Sr is 0.70967. It is indicated that siltcrystal dolostone is formed by burial dolomitization.