To examine the appicability of the converter slag to aggregate, tests were performed for the converter slag specimens which were aged with steam, and the stability of expansion was investigated. The strength of the converter slag was found to he lower than that of the natural aggregate. But the strength of the concrete mixed with the converter slag and the granular slag was increased with an increase of the content of the granular slag. The slump value was larger for the specimen of the converter slag than that for the natural aggregate. The specific weight of the converter slag was decreased with an increase of the aging peroid. The aging time for the converter slag was accessed to be about 48 hours to accommodate the full stability of the expansion. The amount of the steam needed to age one ton of converter slag to full expansion was accessed to be 60 kg. From the regression analysis for splitting tensile strength (t), and flexural strength (f), the compressive strength (c) based on the following formulas were proposed $sigma$t=0.1506 $sigma$c+4.5(kg/cm$^2$) (r=0.876) $sigma$f=O.l537.~+30.5(kg/cm$^2$) (r=0.796)