Ordovician Majiagou paleokarst weathering crust is an important reservoir in Ordos Basin but filling in the early dissolved pores and caves during late diagenesis and burial caused the effective reservoirs to be anisotropic in different areas. On the base of a large numbers of data from microscope observation and geochemical analysis of cores, it is shown that the fillings in pore space are dolomite, calcite, quartz and anhydrite, or assorted of them, in which dolomite-dominating filling is sure to be the most favorable for porosity preservation. Three stages of pore-space filling are identified. It is deemed that the filling is mainly in control of late tectonic transformation in this region.