The determination of free lime in LD and synthetic slags was done gravimetrically by carbonating free lime under the stream of carbon dioxide and water vapor. The results obtained by this gravimetric method were discussed and compared with those of the chemical wet method, i. e. by use of the tribromophenol extraction-atomic absorption spectrum method. Disintegration by the autoclave expansion test was found to be closely related to the free lime concentration, and LD and synthetic slags containing more than 1wt% free lime were found to disintegrate. Mineralogical studies of LD, synthetic and remelted LD slags were carried out in focus of the free lime phase by means of microscopic and EPMA examinations. From these, the free lime phase containing FeO, MnO, and MgO was microscopically found to be hydrated under the atmospheric condition. Free lime grains present in LD slags were clustering, surrounded by the dicalcium silicate phase, but these were dispersed homogeneously after remelting in air. Free magnesia containing FeO, MnO, and CaO was observed in LD slags containing about 10wt% magnesia. This magnesia phase was neither hydrated in microscopic examinations nor disintegrated by the autoclave expansion test.