The microstructure and desorption properties of two types of lightweight fine aggregate（LWFA),sintered fly ash and expanded shale, were investigated. The concept of “internal curing efficiency” was introduced. The effect of LWFA particle size and spacing on internal curing efficiency was also evaluated. The experiment results indicate:the pore volume and size of LWFA are larger than that of the hardened paste; pre wetted LWFA can delay the decrease of concrete internal relative humidity induced by self desiccation and reduce the extent; furthermore, for internal curing concrete, there exists a linear relationship between the autogenous shrinkage and internal relative humidity. When the contents of LWFA by volume of the paste were between 36% and 39%, autogenous shrinkage did not occur within 28 days. The internal curing efficiency decreases as the ratio of LWFA particle spacing to particle size increases. The results show that 100% internal curing efficiency can be achieved if the ratio of LWFA particle spacing to particle size approaches 11.