Spur dikes are hydraulic structures, which are built across the river from river bank towards centerline in order to protect the river bank from erosion, natural development of amending the river side, special protection of local structures, navigation and environmental rehabilitation of river. The main job of these structures is to divert the flow away from the bank and towards centerline of river. However, the velocity will increase and the complicate vortex will be formed around the spur nose and the protection of spur dikes against scouring and their stability is very important. Using riprap is the most common and economical method for spur dike protection.In this study the effect of relative height and length of the submerged spur dikes on riprap stability is investigated. The experiments were carried out in a laboratory flume with 90° bend and R/B=4.0. Six sizes of ripraps including 5.5, 7.9, 11.1, 14.3, 17.9, and 22.2 mm, with three relative spur length (15%,20% and 25% of the flume width) and four dischrge (25,29,32 and 36 lit/s) were used. The results showed that the larger riprap failed for the higher relative spur height. With increasing the relative spur length the riprap stability decreased. Finally an experimental equation was developed based on the relative spur height and length and riprap size, which can be applied for designing of riprap size.