Effects of two types of shrinkage reducing admixtures (SRA1 and SRA2) on shrinkage performance of mortar and concrete as well as properties of both fresh and hardened concrete were investigated. The mechanism of shrinkage reducing was analyzed from the surface tension and ion concentration of pore solution as well as the hydration heat release rate and hydration degree of cement paste. Experimental results indicate that the inhibiting effect of SRA on drying shrinkage is significant before 7d, while it plays a long term inhibiting effect on autogenous shrinkage. The shrinkage reducing mechanism of SRA1 is mainly the synergistic effect of reducing surface tension and K+, Na+ concentration of pore solution of cement based materials. It is interesting that the shrinkage reducing effect of SRA2 is better than that of SRA1, though the reduction of surface tension and K+, Na+ concentration is not as bigger as for SRA1. However, the reduction of bulk shrinkage induced by changing the morphology of the hydration products at early age due to retardation of hydration may be the main reason for the shrinkage reducing of SRA2.