Calcareous interbeds develop widely in Chang 8 tight sandstone reservoirs, Zhenjing Area, Ordos Basin. This paper reported the lithology, electricity, distributing characteristies of the calcareous interbed and the relationship with the distribution of natural fractures based on outcrops, cores and logs data. Studies have shown that calcareous interbeds have the following electrical characteristics. Natural gamma ray log is low. Longitudinal wave offset and transverse wave offset time are also low. The value of density logging is high and induction logging is obvious high. Electric imaging logging has the characteristic of clear light yellow (light). Good development of calcareous interbeds presents at the middle or bottom of underwater distributary channel sand body and at the top or middle of mouth bar sand body in delta front. Besides, calcareous interbeds are distributed in well sorted, grain supported fine sandstones and contact zones of mudstones and sandstones. There are mainly high angle tectonic fractures in calcareous interbeds with little nearly horizontal diagenetic fractures. Calcareous interbeds are brittler than non calcareous cementation tight sandstone. In consequence there are more natural fractures and the majority of them are filled with calcite taken as invalid. The thickness and lithologic interface of calcareous inerbeds determine the longitudinal height of fractures. All the achievements of this thesis are significant to study the distribution patterns of natural fractures and the effect of fractures on water flooding development.