Abstract Modern trends in the pipe-rolling industry have been to produce thinner-walled pipe with higher-strength steel. However, as many of the existing conventional pipe-rolling mills were designed to accommodate heavier-walled pipe, the forming of the lighter sections can lead to forming difficulties. As the sheet material, or skelp, becomes thinner, its bending stiffness decreases, increasing the occurence of edge instabilities that can reduce the overall weld quality. Recent research has developed analytical and numerical models for the prediction of the strain history in the roll-forming process, but there has been little work done on the skelp edge instability phenomenon. This paper presents an investigation of skelp edge instability in the roll forming of ERW pipe. A model has been developed to predict the skelp edge instability, based on prediction of local axial deformation, during the forming process. The model reveals the relationship between the local axial deformation and the mill set-up parameters, which can be used to assist in adjusting the mill to eliminate skelp edge instability.