The properties and void structures of foamed concretes prepared from portland cement, fly ash, steel slag and foam agent were investigated. The results show that substituting cement by steel slag resulted in little change in dry bulk density, but reduced the compressive strength; on the other hand, when cement was substituted with fly ash, though the dry bulk density decreased, the compressive strength increased as the proportion of substitution increased from 30%to 45%. Increasing the volume fraction of the foam decreased greatly the dry bulk density and the compressive strength, and reduced the thermal conductivity as well. Image analysis on the air void structure shows that the larger air void fraction, the larger average void size and lower void roundness. There is a close correlation between the average mean Feret diameter and the average shape factor.