The upper limit of Froude number for the undular jump formation has been investigated by many researchers, but a general agreement has not been obtained. The formation of undular jumps is affected by the inflow Froude numbei the development of the turbulent boundary layer, the aspect ratio, and the Reynolds number. In this paper, the effect of the Reynolds number and the aspect ratio on the undular jump formation has been clarified, and a general expression for the upper limit of Froude number has been established. The minimum scale of model required to apply the experimental results to a prototype design can be determined as Re≥65000. Also, if the aspect ratio is larger than 10, the flow condition of undular jumps can be regarded as a two-dimensional flow.