The author have studied the sedimentary environment and formation mechanism of nodular limestones in the Middle Ordovician rhythms of Keping Area,Xinjiang.In these limestones,thin layers of limestone are interbedded with mudstones.According to detailed field work as well as cathode fluorescence and dyeing under microscope,these nodular stones can be classified into three types on the basis of their structures,namely,mas- sive limestone,wavy limestone and lentoid limestone.Based on a synthetic analysis of elements with XRD,C and O isotopes with MS,and a study of mineralogy,petrology and paleontology,it is believed that they were formed in a sedimentary environment of shelf to deep basin.The carbonate-argillite rhythms were formed under the cyclic influence of surface currents,ocean bottom currents and source of sedimentary substance.Nodular limestones were formed in the early period of sedimentation-diagenesis and their formation mechanism is as fol- lows:under the periodic influence of the surface currents and the deep currents,the massive limestone was formed around the nucleus through coagulation,multiplication and cementation during the quiet period.The wavy limestone resulted from periodic washing of the carbonate marl by the salt-nonsaturated deep water during the period of syngenesis.The lentoid limestone was formed by the differential pressures between the water body and the overlying sediments.Every kind of nodular limestones had its main influential factor.