During the commission period of flue gas desulfurization(FGD), a considerable amount of scale was formed in GGH at 2 bituminous coal power plants which were named as T and H. Various attempts have been studied to identify the causes of the scale formed in GGH, for example, concentration analysis, surface morphology, and crystal structure of scale, gypsum and ash. According to the results of stereozoom microscopy, GGH scale was severely harder than absorber scale. In addition, Si, Al and Ca were observed from concentration analysis, and quartz and mullite crystal structure were identified from XRD, and very fine(size: less than 1 μm) and spherical-shaped particles were shown from SEM. In addition, the size of gypsum particle observed from XRD and SEM was 1-10 μm, and this size was very minute compared to the typical size of gypsum(size: about 40 μm) particle produced in FGD process. Although small differences were found between 2 power plants, the causes of scale formed in GGH were deduced that some saturated gypsum slurry was scattered in the reactor of FGD and that the slurry droplets were not eliminated properly in M/E. And then they arrived in GGH water or mist in them was eliminated, and finally formed the scale in GGH. The scale in turn got entangled with coal ash of untreated flue gas from boiler, and so the formation of scale was accelerated.