EPS (expanded polystyrene) is one of the most used building materials for insulation that is favored by itsexcellent heat insulation, economical efficiency and lightweight characteristics. However, EPS is vulnerable to the fire andproducing large amount of toxic gases in case of fire. Therefore, ultra-lightweight geopolymer which can replace EPS isfabricated by using IGCC (integrated gasification combined cycle) fused slag and Si sludge as raw materials and thepossibility of replacement on ultra-lightweight geopolymer for EPS as an insulation building material was evaluated in thisstudy. Ultra-lightweight geopolymer can be fabricated with the pulverized IGCC fused slag having low carbon content anddensity, compressive strength, thermal conductivity were 0.064 gcm3, 0.04 MPa, and 0.072W/mK, respectively. The thermalconductivity of ultra-lightweight geopolymer is 1.5~2.0 times higher than that of EPS suggested in the KS M 3808; however,the thermal conductivity value of geopolymer is meaningful and competitive to that of EPS in the market. Therefore, ultralightweightgeopolymer can be applicable to the building material for thermal insulation purpose and have an enoughpossibility to replace EPS in the future because it is not only much safer than EPS in case of fire but also it can befabricate by using waste materials from the industry.