Abstract: There exist abundant iron ooids at the bottom of the Middle Devonian Yangmaba Formation in the Longmenshan area. In order to analyze the genesis of the iron ooids, their characteristics were studied by means of thin section and scanning electron microscope based on the detailed measurement of the Shiligou Profile in Ganxi village. The analysis results show that there are various types of ooids in Middle Devonian Yangmaba Formation of Longmenshan area. The ooidal cores include four types:quartz clastic particles, crinoid fragments, black hematite and calcite. The outer laminas of ooids can be divided into three types:alternate dark and bright one, uniform-colored one and chlorite-fragmented one. The iron ooids of Middle Devonian Yangmaba Formation in the Longmenshan area have an important significance to the ancient environment. The silty ooids were formed in the open environment nearshore shallow water under oxidation condition, and iron ones and chlorite ones with thin layers were deposited in the relatively closed lagoon and residual ooids beach of shallow seashore.