The ultrafine grinding of white cement and its hydration were studied. The ultrafine grinding parameters, such as crushing pressure and feeding pressure of jet mill in a lab scale were optimized. The influence of the particle size distribution of ultrafine white cement and the usage of water reducer on the standard thickness and early age strength of white cement paste was investigated. The results indicated that the ultrafine white cement with d50=4.62 µm and d90=10.48 µm can be obtained when the crushing pressure was 0.7 MPa and the feeding pressure was 0.4 MPa. The standard thickness of white cement paste decreased from 30.5 % of the raw material to 26.0% of the ultrafine white cement (d50=4.42 µm, d90=10.26 µm). The use of 2% naphthalene-based water reducer can reduce 15.4% water consumption. The 3d compressive strength of pure ultrafine white cement sample reached 174% of that of raw white cement. The 28d compressive strength of ultrafine white cement with 3% naphthalene-based water reducer added was 22.9% of that of pure ultrafine white cement. A large number of needle-shape nanometer ettringite (AFT) was generated in the early stage of hydratration and layered C-H-S was formed with the decreasing of the amount of AFT in the later stage of hydratration. It’s supposed that the formation of the large number of needle-shape nanometer AFT was related to the reducing of the particle size of C3A during the ultrafine grinding and the no adjustment of the amount of the added gypsum, which resulted in the faster hydration of C3A.