Abstract: The grainy denudation is one of the most important modes of mechanical denudations, which has a great influence on the heat shield of fibrous composites. The formulas for fibre’s shape are obtained by a steady-state analysis. Based on the formulas, a mechanical model is set up to solve the grainy denudation problem of fibrous composites. Using the Mohr criterion, an estimate can be made for deciding if grainy denudation occurs. According to the estimate, the differences between fibre and bulk, the outer fluid velocity and the fibre’s dimension would markedly affect the denudation properties of fibrous composites.