Some physico-chemical studies have been conducted to utilize steelmaking slag for the fixation of CO2 with the multiplication of the marine phytoplankton. The targets of present work are to reveal the morphology of the industrial steelmaking slags, the control of crystal size of each phase precipitated in the slag by heat treatment, and the dissolution behavior of elements from the slags into the artificial seawater. The dissolution limit of each element has been thermodynamically discussed based on the stability diagram.