Abstract: Hot modification at electrical arc furnace(EAF) slag tapping process is a kind of new process, which can utilize sensitive heat of molten slag, recovery iron of slag and improve its cementitious activity. EAF slag was modified into the slag with basicity from 0.9 to 1.7 in hot stage. Effects of basicity on cementitious activity of the slag were studied. Results indicate that the activity reduces at first, increases then and declines at last with increasing basicity of modified slag from 0.9 to 1.7. Meanwhile, slags with the basicity of 0.9 and 1.3 have higher cementitious activity. The slag with basicity of 0.9 resultes from the high glass content characterized by exothermic peak area of DSC curves. The slag with basicity of 1.3 is due to the high reactive activity determined by low polymerization degree of glass phase which is tested by Raman spectra. However, the slag with basicity of 1.3 satisfies the modification process by using the sensitive heat of molten slag only and its flexural, and compressive activity indexes are 107% and 91%, respectively.