Scale formation in surface and subsurface oil and gas production equipment has been recognized to be one of the major operational problems. It has also been recognized as the major cause of formation damage either in injection or producing wells. Scale contributes to equipment wear and corrosion and flow restriction, thus resulting in a decrease in oil and gas production. This article presents the predicting of calcium sulfate scale tendency of formation water, injection water, and mixing of injection water with formation water at different temperatures. The experimentally measured chemical analyses of formation water and injection water were used by OLI ScaleChem software to determine the tendency of scale formation. This software has been applied to investigate the potential of calcium sulfate scale precipitation in Iranian oilfields, either in onshore or offshore fields, where water injection is being performed for desalting units' water disposal purpose or as a method of secondary recovery or reservoir pressure maintenance.