The flow on high spillways has quick velocity, thus it often creates cavitation and cavitationary erosion on the surface. This paper presents the method to check the cavitation and cavitationary erosion caused by local roughnesses on the surface of the spillways. The calculation of the flow regime on the Lai Chau spillway shows the range of cavitation and cavitationary erosion resulting from different levels of surface roughnesses. Although the spillway is made of high intensity concrete and the surface is constructed at the highest level possible, cavitation and cavitationary erosion is unavoidable. In order to minimize this, some special measures are needed.