Recently, the ground granulated blast-furnace slag(OBFS) with higher fineness than before has been produced by the aid of the developement of grinding teehnoldgy, and widely used as one of structural materials. This paper shows the effects of steam curing (SC) on the properties of concrete used with high fineness GBFS. The results, compared to ordinary portland cement(OPC), were as follows. (1)The compressive strength of SC concreteused with GBFS decreased with increasing the amount of GBFS and increased with increasing the fineness of GBFS.(Fig. 1) (2)Inthe same SC condition, the compressive strength of SC concrete used with the GBFS of the content of 70 percent and the fineness of 6000cm2/g was larger than that of OPC concrete at one day.(Fig. 1) (3)The diffusion coefficiency of GBFS concrete was less than that of OPC concrete.(Fig. 5) (4)The resultsof porosimetryshowed that GBFS concrete was more in the amount of relatively small pore, less in the average radius of pore and a little more in the total amount of pore than OPC concrete. (Fig. 8, Fig. 7) (5)The amount of Ca(OH)2 decreased with increasing the content and fineness of GBFS. (Fig. 8)