Abstract: The slag weight loss during roasting for electroslag remelting was investigated in laboratory via thermogravimetry. The relationship between roasting temperature and the content of hydratable components in slag was studied, especially the relationship between slag bed thickness and roasting time and that between roasting temperature and time under zero-gravity conditions, with several equations given by numerical fitting. The experimental results were analyzed through solid-phase thermal decomposition mechanism and indicated that in the earlier stage the thermal decomposition is dominated by nucleation and growth of crystal nucleus, and in the later stage it mainly depends on the diffusion controlled reaction. Such results will provide the directions to the roasting process of slag.