The simulation technology for virtual design of a decurler device in electrophotography is developed. A paper curl is a phenomenon in which arc-like residual deformation occurs, and it degrades publishing quality and printing productivity. To reform the paper curl, decurler devices are installed in electrophotography products. In the decurler devices, paper is bent in large curvature against the direction of the curl to be reformed. Reformation amount of the curl is affected by decurler parameters including radius of rollers and nip load. In addition, amount of the curl to be reformed varies with paper brand, type of fusing system and image density. For these reasons, much effort and many resources are required for designing decurler parameters. To clarify the mechanism of the curl reformation, measurements of curl amount in various decurler conditions are carried out, and it becomes apparent that the paper curl is dependent on plastic and viscous characteristics of paper. By calculating history of stress-strain state during transportation through decurler devices with visco-elasto-plastic model which reflects characteristics of the paper, prediction of curl amount in high accuracy becomes possible. By utilizing this technology for virtual design of decurler devices, reduction of the development period to one fifth was accomplished.