The study of the first alkali silica reaction (ASR) case in Thailand was conducted on fluoresced gel under UV light technique. It was done on ten random cored samples from different cracked structures of ten year-aged project and the other three cored samples from no-crack structure of one building and two bridges, ages older than 40 years. It was study on areas of suspected ASR gel in the interface zone (ITZ), aggregates and paste in the vicinity. The results of reflected area indicated the evidence of ASR gel at aggregate’s rim and the nearly areas as well as in some cracks in the matrix of 9 in 11 samples of the first set. The amount of reactive coarse aggregates was found in the range of 5-90% of total coarse aggregate content, which was supported by the expansion results of same cored samples from previous study. The results of this study indicated that the fluoresced gel technique provided the satisfied results which confirmed the accelerated expansion test from previous study. They can be used as an effective means for ASR screening test of existing structures both in the field and in laboratory.