Abstract: Nano material is able to accelerate the hydration of cement. Nano silica has recently been widely studied for its use in well cementing. Nano silica accelerates the hydration of cement excessively because of its high specific surface area. Calcium silicate hydrate seed crystal, an artificially synthesized nano calcium silicate hydrate, can accelerate the hydration of cement through nucleation. Using Ca(NO₃)₂·4H₂O and NaSiO₃·5H₂O as raw materials, a calcium silicate (C-S-H) seed crystal was developed through precipitation reaction, and study on the use of C-S-H as oil well cement accelerator was conducted. The heat of hydration and hydration rate of cement were measured unintermittently at 10℃ for 96 hours with Tam air isothermal calorimetry. It was found that with an increase in the concentration of the seed crystal, the induction time of cement hydration was notably shortened, and the amount of heat released was increasing. The developing progress of the strength of the set cement, measured with an ultrasonic strength analyzer in the study, indicated that the use of seed crystal notably shortened the time required for the cement slurry to develop strength, and the rate of strength development of the set cement increased obviously. The more the seed crystal used, the faster the development of the strength of set cement. It was also found that the addition of the seed crystal did not notably increase the viscosity of the cement slurry. From these studies it is understood that the C-S-H seed crystal is able to accelerate the hydration of cement, increase the early strength of set cement and shorten the induction time for gel strength development. It is a potential low temperature accelerator.