It is important to clarify the formation, break and flow of river ice on frozen rivers. The aim of this study is to clarify ice break and flow phenomena during river ice breakup, as these phenomena are the initial stages of ice jam. With regard to the break of river ice, we developed a method of judging ice break by comparing the stress caused by the flowing water and flowing ice, and the allowable bending stress of the river ice. The ice flow is expressed using the continuity equation that considers the change in amount of ice, and the equation of motion that considers the effect of the flowing water. This one-dimensional mathematical model showed that although there is a problem with the reproducibility of ice jam, it is possible to reproduce ice break and flow phenomena by comparing the freezing conditions from the results of the calculations, with those of aerial images of the actual river.