An experimental investigation was conducted to determine the effect of a nonuniform pier on the stability of riprap placed around bridge piers. A nonuniform pier is one of which the cross-sectional dimension varies over the length of the pier and comprises a cylinder of diameter bp placed on a larger diameter of foundation bf. and the stability of riprap are significantly influenced by the height of foundation z. The critical height of foundation is defined as the height of foundation which has the same critical velocity to that of uniform pier without foundation, and it was found to be zc=0.8bf. For z<zc, the foundation increases the stability of ripraps by intercepting downflow and by acting as a foundation upstream extension. For z<zc, the stability of riraps is reduced due to the enlargement effect of foundation and downflow along the foundation itself. The effects of nonuniformity of piers are presented and design equations are suggested.