Abstract: CNOOC has in recent years drilled several ultra-deep water wells in South Sea of China, the deepest water depth has been more than 2,000 m. Extremely low temperature has imposed serious challenges to the strength of set cement. Two new low temperature early strength agents, ACC and NS, have been developed to deal with well cementing challenges in deep water drilling. Studies on the effects of ACC and NS on the early strength of cement slurry at low temperatures showed that the optimum concentrations of ACC and NS was 8% and 3%, respectively. At low temperatures between 3℃ and 15℃, ACC and NS can improve the 24-hour and 48-hour compressive strengths of set cement. Continual monitoring of cement slurries for 24 hours at 15℃ using hydration heat analyzer and ultrasonic strength analyzer showed that, compared with blank sample cement slurry and original early strength additives, addition of ACC and NS obviously increased the amount of heat of hydration and hydration reaction rate of cement. The rate at which the strength of the set cement was developing was obviously increased, while the transition time of the gel strength of the cement slurry was obviously shortened. ACC and NS have been used satisfactorily on the well WN-XX in China's South Sea.