Karstification changes the chemical composition of original carbonate sediments／rocks； therefore geochemical characteristics may serve as important grounds for identification of palaeokarst。The first submember of the fifth member of the Lower Ordovician Majiagou Forma tion(MSl)in Xingxian，Shanxi Province，which was deposited in shallow and confined epiconti- nental environments with low energy and high salinity，consists of carbonate rocks modified by palaeokarstification for a long period： In the light of contents of CaO，MgO，SiOz， AlzOs， Fe20”Na20，K20，Co，Ba，Zr，Zn，V，Th，Ti，Si，P，Ni，Mn，La，Ce，Cu，Cr，Li，Se，U and B of 1 1 8 samples from M51，it may be concluded that sedimentation，karstification，and burial diagen【esis controlled the geochemical composition of carbonate rocks，of which karstifica- tion plaved the most important role．As a result of the combined effects of sedimentation，karsti— fication。and burial diagenesis，there are high values of Si02，A1203，Fe203，Na20，Ba，Ti，Ce and Li，higher values of Th，Ni，Cu，Cr and La，and low values of MgO，K20，Sr，P and Mn in the M5，．