Williams syndrome was first reported in 1961, it is characterized by distinct facial changes, growth deficiency, mental retardation, congenital heart defect (particularly the supravalvular aortic stenosis), associated at times with infantile hypercalcemia. The diagnosis of this disease relied on phenotype, after recent acknowledgement of its genetic basis on the 7th chromosomal locus q11.23 involving elastin gene on the PCR. Recently, commercial probe of elastin gene for FISH was supplied and diagnosed this disease easier. Using PCR has some benefits concerning whether the origin of the genetic defect is maternal or paternal. The diagnosis of this disease is difficult due to low sensitivity, below 50%. But FISH is widely used because it is faster with high positive predictibility. We report Williams syndrome diagnosed by FISH with a brief review and related literatures.