Blast furnace slag is recycled as high value-added material, while steel slag is difficult to recycle or is recycled as low value-added one relatively due to its expansion collapsability. It``s property is caused by the high content of free lime and instable steel oxides. Recently a innovative rapid cooling method for melting steel slag has been developed in Korea, which reduced free lime content to minimum level and increase the stability of steel oxides. However, research on the long-term stability of it is not insufficient so far. Therefore, this study, focused the electric arc furnace oxidizing slag in the steel slag, aims to investigate the properies of the rapidly cooled steel slag aggregate, its long term volume stability and the engineering strength of mortar using it as a fine aggregate. This sudy result indicated that it was possible to use as concrete aggregate because the volume change of the steel slag appeared to be stable