The Lujing uranium ore field which is located in the southwest part of Taoshan- Zhuguang metallogenic belt is one of the most important granite type uranium ore field in South China. This ore field is dominated by the uranium mineralization related cataclastic alteration. The Xiaoshan uranium deposit is one of newly discovered cataclastic alteration granite- type deposits during recent years. Chloritization is the most important alteration type and prospecting indicator of the cataclastic alteration granite- type uranium deposits. However, the course of chloritization and its relationship with uranium mineralization has not been paid much attention. In this study, the mineralogy of chloritization from hydrothermal zone that was uncovered by drill core ZK- 1 in Xiaoshan deposit has been carried out. The following conclusions have been reached in this study. ①There are four types of chlorites, including chlorite of biotite alteration, chlorite of feldspar alteration, chlorite vein/veinlet filling in fissures, chlorite closely associated with uranium minerals. ② The chlorites are mainly plotted in the brunsvigite area, partly plotted in the pycnochlorite. ③ The formation temperature of chlorites varies from 213.5℃ to 249.8℃, at an average of 233.4℃, suggesting the chlorites were formed under medium to low temperature conditions. ④ The chlorites were formed under the reduced condition with low oxygen fugacity and high sulfur fugacity through two formation mechanisms of the dissolution- precipitation and the dissolution- migration- precipitation. The uraninite, monazite and xenotime are redissolved to form coffinite- thorite. ⑤ The chloritization has changed the properties of wall rocks, the occurring state of uranium in rocks and physicochemical environment and promoted the activation, migration and precipitation of uranium.