In this study, the strength characteristic of lightweight aggregate concrete made of BFS hardening by chemical reaction of alkali activate admixture was analysed and the results are as below. Air content and slump value appeared decrease by increasing the amount of alkali activate admixture and replacement of Meta Kaolin(MK). It is considered that the decrease is because of increase of the binder granular amount and high fineness of MK. Proportion design by replacement ratio of alkali activate admixture needs more studies for improving workability. The compressive strength was measured over 28MPa at age 28days when 0%, 10%, 20% of MK replacement. This confirms the possibility of use of alkali-activated lightweight concrete as lightweight concrete. And as MK reacted with more amount of alkali activate admixture than BFS, the optimum mix proportion of alkali activate admixture needs to be established for improving compressive strength.