The study of Chang 8 sandstones from Upper Triassic Yanchang Formation in Zhenjing area, Ordos Basin, was undertaken to unravel the characteristics of chlorite minerals, such as the type, occurrence, formation time and origin and so on. The chlorite minerals in Chang 8 sandstones present as three different occurrences: terrigenous clastic chlorite, authigenic chlorite and alteration chlorite, in which the authigenic chlorite containing grain-coating chlorite, pore-lining chlorite and pore-filling chlorite, with the pore-lining chlorite as the main type. The formation sequence of different chlorite follows: grain-coating chlorite→pore-lining chlorite→pore-filling chlorite, with the terrigenous clastic chlorite as the synsedimentary product and alteration chlorite forming in any diagenetic time. The crystal chemistry, dimensional distribution pattern and origin of each type chlorite are different, controlled by the sedimentary environment, composition and texture of sandstones, the resource of Fe and Mg, temperature and pore fluid attribute. The terrigenous clastic chlorite formed in sedimentary period is dominated by provenance condition and mainly distributes in the interdistributary with low hydrodynamic force. It has the highest Fe、Mg、Mn、AlⅣ, and lowest Si、Ca、AlⅥ. The alteration chlorite is formed by the chloritization of Fe- and Mg-rich clastic fragments, with a similar distribution pattern like them. It has higher Fe、Mg、AlⅣ, and less Si、Ca、AlⅥ. The authigenic chlorite has the lowest Fe、Mg、AlⅣ, and highest Si、Ca、AlⅥ. The pore-lining chlorite has higher Fe、K and lower Ca、Mg than pore-filling chlorite, moreover, from the detrital grain edge to the pore center, the contents of Fe、Mg、AlⅣ and Sum (VI) become much higher, and the contents of K、Si、AlⅥ have a converse variation tendency. The grain-coating and pore-lining chlorite developed during the syngenesis and eodiagenesis respectively are principally controlled by depositional environment, and mainly distribute in the subsea distributary channels and mouth bars, with the Fe and Mg materials deriving from the dissolution of flocculated, clay-like and Fe- and Mg-rich materials. The pore-filling chlorite took place in the late A stage of mesodiagenesis is dominanted by the structure of sandstones, and chiefly develops in the sandstones with coarser size and better pore throat structure, with the Fe and Mg coming from the discharge of mudstones.